Amveco Medical Grade Isolation Transformer

MEDICAL POWER & ISOLATION TRANSFORMERS 50/60Hz

Lifetime warranty


Customized standards or full custom designs are available at nominal or zero additional costs.

WINDING CONFIGURATIONS WITH COLOR CODES

Quad Primaries: 100V, 120V, 220V, 240V - 50/60Hz
Multiple primaries must be connected in series or parallel.



 

NOTE:
(suffix) SS = (Single Secondary) Secondary 1 only
(suffix) DS = (Dual Secondary) Secondary 1 & 2

Units rated below 1000VA come with metal disk and insulating pads.
Units rated 1000VA and larger are center potted.
Part numbers MT100 through MT5000 carry full TUV BAUARTMARK.
 

The values given are typical values.
Technical data subject to change without prior notice.
 

Below is a article written by the Medical Device & Diagnostic Industry Magazine MDDI Article Index

ITE Power Supplies and Medical Equipment
There are potential pitfalls in using ITE power supplies in medical devices. Knowing applicable standards will help you avoid mistakes.
Frank O'Brien
A large OEM power-supply industry has sprung up to meet the voracious appetite of the information technology equipment (ITE) market for global, high-volume, reliable, cost-effective power supplies. These OEM power supplies are a tempting option for medical equipment manufacturers as well. However, medical equipment regulatory requirements must also be met—a crucial design factor that demands caution when one considers an off-the-shelf ITE power supply for a new device.

In general, ITE and medical equipment share connections to supply mains and the provision for input/output data connections. In both cases, it's anticipated that users or operators will have access to equipment surfaces, and sometimes to data circuits. The primary hazard associated with this type of access is electrical shock. The power supply plays a fundamental role in protecting against this hazard.
 

The primary and obvious difference between ITE and medical equipment is that medical equipment is intended to diagnose, treat, or monitor a patient. Therefore, medical equipment normally comes in intentional contact with the patient: equipment surfaces within the patient vicinity can be contacted by the patient, or contacted by an operator who is also contacting the patient. As patients can be unconscious, connected to multiple equipment, or have open skin regions, the risk of electric shock can be great. Again, power supplies play an important role in protecting against an electric shock hazard in such settings.

The primary and obvious difference between ITE and medical equipment is that medical equipment is intended to diagnose, treat, or monitor a patient. Therefore, medical equipment normally comes in intentional contact with the patient: equipment surfaces within the patient vicinity can be contacted by the patient, or contacted by an operator who is also contacting the patient. As patients can be unconscious, connected to multiple equipment, or have open skin regions, the risk of electric shock can be great. Again, power supplies play an important role in protecting against an electric shock hazard in such settings.

The intent of this article is to:

Examine the safety requirements for ITE and medical equipment.
Compare their fundamental approaches to protection against electric shock.
Identify areas in which ITE power supplies may require further evaluation.
Examine some available options.
Outline ideal safety specifications for medical power supplies.
SAFETY REQUIREMENTS

Information Technology Equipment. Most major world markets require IT equipment to provide reasonable protection against injury for users and damage to property. In the United States, regulations from OSHA and FCC must be met. In Europe, the Low Voltage Directive and the Electromagnetic Compatibility (EMC) Directive must be satisfied. Compliance with the international safety standard, Safety of Information Technology Equipment, IEC 60950, 3rd edition (1999-04), is the generally accepted way to satisfy the safety requirements of markets around the world. Additional regulations exist for EMC. (EMC requirements are outside the scope of this article. However, the internationally accepted standards that largely satisfy global regulations are based on CISPR 22 and CISPR 24.)

Within each national or regional market, the international standard must be considered together with any published deviations that take into account local installation codes and expectations of safety. In the United States and Canada, national deviations are contained in the binational standard, CSA C22.2 No. 950-95/UL 1950, 3rd edition (1995-07). This standard is based on IEC 60950, 2nd edition (1991) + Amendment 1 (1992-02) + Amendment 2 (1993-06) + Amendment 3 (1995-01) + Amendment 4 (1996-07). UL and CSA are planning to issue an updated binational standard, based on IEC 60950, 3rd edition, by the first half of 2000. In Europe, regional deviations are contained in EN 60950 (1992) + Amendment 1 (1993) + Amendment 2 (1993) + Amendment 3 (1995) + Amendment 4 (1997) + Amendment 11 (1997).

Medical Equipment. Most major world markets regulate medical equipment. In the United States, the Federal Food, Drug, and Cosmetic Act (and succeeding acts) requires that all medical devices be "safe and effective," and FDA recognizes consensus standards as a means to support a declaration of conformity (new 510(k) paradigm, "abbreviated 510(k)"). FDA lists IEC 60601 + national deviations (UL 2601-1) as a recognized consensus standard. In Europe, the Medical Devices Directive (93/42/EEC, Article 3) requires medical devices to meet the "essential requirements." Compliance is presumed by conformity to the harmonized standards in the Official Journal of the EC (93/42/EEC, Article 5). IEC 60601 + regional deviations (EN 60601) is a harmonized standard. Similarly, IEC 60601 forms the basis for national medical equipment safety standards in many countries, including Japan, Canada, Brazil, Australia, and South Korea.

IEC 60601 is a series of standards. The basic standard containing the core safety requirements for all electrical medical equipment is Medical Electrical Equipment—Part 1 General Requirements for Safety, IEC 60601-1, 2nd edition, (1988-12) + Amendment 1 (1991-11) + Amendment 2 (1995-03). There are collateral (horizontal) standards, which supplement the core requirements by providing technology-related safety requirements. The naming convention for collateral standards is IEC 60601-1-xx. Technologies addressed by collateral standards include medical systems, EMC, x-ray radiation, and programmable systems. There are also particular (vertical) standards, which supplement the core requirements by providing device-specific safety and performance requirements. The naming convention for particular safety standards is IEC 60601-2-xx. Specific devices addressed by particular standards include RF surgical devices, ECG monitors, infusion pumps, and hospital beds. There are approximately 40 Part 2 standards. Although most of these also include essential performance requirements, the trend is to move to another (-3-xx) series of standards for essential performance.

Within each national or regional market, the international standard must be considered together with any published deviations that take into account local installation codes and expectations of safety. In the United States, the national deviations to the core safety requirements are contained in the standard UL 2601-1 (1997-10). In Europe, regional deviations to the core safety requirements are contained in EN 60601-1 (1991-01) + Amendment 1 (1994-06) + Amendment 2 (1996-03) + Amendment 13 (1996-07).

For EMC, the internationally accepted standards that largely satisfy global regulations are in IEC 60601-1-2 (1993-04), and are based on CISPR 11, CISPR 14, and IEC 60801. For some equipment, the FDA reviewer guidance document for premarket notification (510(k)) submissions contains EMC recommendations that are not entirely represented by IEC 60601-1-2.

Since the United States and European markets are the largest ones for medical equipment manufacturers, only the national and regional standards relevant to those markets were cited above. The remainder of this article addresses the generic IEC 60950 and IEC 60601-1 requirements. All references are to IEC 60950, 3rd edition (1999-04) and IEC 60601-1, 2nd edition (1988-12), including all amendments. U. S. and European deviations do not amend the requirements referenced below.

COMMON SAFETY PHILOSOPHY

When designing a product to provide reasonable protection against injury and property damage, how safe is safe enough? This is the question that safety standards address. IEC standards are consensus documents, which define the minimum design requirements.

As technology advances, device usage evolves, and other factors change, a standard can become out of date and its requirements no longer appropriate. Thus, the use of standards does not preclude the need to conduct a risk analysis on products.

Two Levels of Protection. According to IEC 60950, Sub-clause 1.3.1, "Equipment shall be so designed and constructed that, under all conditions of normal use and under a likely fault condition, it protects against risk of personal injury from electric shock and other hazards, and against serious fire originating in the equipment, within the meaning of this standard." ITE must be "safe" in normal and likely fault condition. There are a number of ways to meet this fail-safe requirement, the most common of which is to design in a backup level of protection. When employing this strategy, it must also be unlikely that the failure of the first means of protection will go undetected or tolerated by the user before a likely fault of the backup protection occurs. To summarize, a common strategy for designing ITE is to design in two levels of protection.

In some situations, however, two levels of protection might not be considered a reasonable level of protection against injury or property damage. For example, developers of safety requirements for aircraft, nuclear power plants, or missions to Mars may have other ideas about what constitutes a reasonable amount of protection. However, the ITE standard defines two levels of protection as sufficient for ITE.

In a similar manner, IEC 60601-1, Clause 3.1 states that "Equipment shall, when transported, stored, installed, operated in normal use, and maintained according to the instructions of the manufacturer, cause no safety hazard which could reasonably be foreseen and which is not connected with its intended application, in normal condition and in single-fault condition." As with ITE, medical equipment must be "safe" in normal and likely fault condition, and a common strategy for designing such equipment is to design in two levels of protection.

Table I. Correlation of terminology for IEC 60950 and IEC 60601-1.

COMMON TERMINOLOGY

There is generally a one-to-one correlation of IEC 60950 and IEC 60601-1 terminology. Table I summarizes this mapping, and provides the basis to discuss both standards with a common language. In this article, the medical terminology will be used.

PROTECTION AGAINST ELECTRIC SHOCK

At established threshold levels, an electrical current passing through the body can cause electric shock. This current depends on the voltage and the body impedance.

Fundamentally, the basic strategies to limit the accessible current, and therefore protect against electrical shock, are to place high impedance (insulation) in the current path, or limit the accessible voltage (earthing or grounding).

 

Table II. Levels of protection (LOPs) against electrical shock.

As mentioned previously, both IEC 60950 and IEC 60601-1 require two levels of protection against electrical shock. Both IEC 60950 and IEC 60601-1 define insulation and earthing (grounding) terminology to denote their role with regard to levels of protective provided. Table II summarizes the terminology.



Figure 1. (a) Basic insulation plus protective earthing, (b) basic plus supplementary insulation, and (c) reinforced insulation.

Knowing that the goal is two levels of protection, it becomes clear that the combination of basic insulation and protective earthing satisfies this requirement. If the basic insulation fails, the user has access to a protectively earthed surface. If the protective earthing supply connection is broken, the basic insulation provides separation (high impedance) between the live part and the user. This is illustrated in figure 1a.

Similarly, basic and supplementary insulation will provide two levels of protection. By definition, this is considered double insulation. This scheme is illustrated in figure 1b.

Reinforced insulation is a single system that has been shown to provide the equivalent of two levels of protection, as shown in figure 1c. By definition, it is considered more than a single fault for reinforced insulation to fail.

Looking at the levels of protection provided by the insulation types, one might wonder if two means of basic insulation could be considered as providing two levels of protection. However, basic insulation by definition is not intended to be a backup means of protection. This role is reserved for supplementary insulation (or protective earthing).




 

Figure 2. Protective earth (1 LOP).

For earthing to provide a level of protection against electric shock, it's important that the earthing connection have a sufficiently high current-carrying capacity and low impedance. In the event of a failure of basic insulation, the earthing connection must remain intact at least until the branch protection clears (high current), and the accessible surfaces must remain at or near earth potential (low impedance). Figure 2 illustrates this principal. Earthing that has not been evaluated for these properties is by definition functional earthing. Earthing provided for EMC shielding purposes is an example of earthing that may not need to be evaluated as protective earthing.

When considering the suitability of insulation, it's important to consider the voltage stress the insulation is subjected to under normal use, which is defined as the reference voltage. IEC 60601-1, Clause 20.3 specifies, "For insulation between two isolated parts or between an isolated part and an earthed part, the reference voltage (U) is equal to the arithmetic sum of the highest voltages between any two points within both parts." In the case in which both parts share a common earth reference, the reference voltage is the higher of the voltage in either of the two parts. With IEC 60950, these same requirements must be considered as well as voltage peak values. In the case of switching power supplies, repetitive peak voltages are common. (The consequence this can have on air clearances required by IEC 60950 is presented in more detail when air clearances are discussed below.)

Insulation diagrams (sometimes called isolation diagrams) are used in product design to graphically:


Identify insulation types, reference voltages, and earthing types.
Determine required creepage, clearance, and transformer-layer insulation thickness (physical requirements).
Determine required dielectric-strength values (test requirement).
Identify alternative constructions.
Convey design criteria to purchasing staff, vendors, and others.
Figure 3 illustrates a simple insulation diagram for a product with a supply mains connection, a protectively earthed (PE) enclosure, and a SIP/SOP (ITE SELV) data connection. The mains circuit is considered a mains part (MP); the SIP/SOP circuit is considered a live part (LP). Bridging the mains part and the live part is the power supply (P/S). Shown is an insulation and earthing scheme that provides two levels of protection against electric shock: from MP to PE is basic insulation, and from MP to LP is double or reinforced insulation. The SIP/SOP is tied to earth (FE), as is common with ITE. This is acceptable in medical equipment in cases when separation requirements in IEC 60601-1 Clause 17(g)2 and accessibility requirements in Clause 16(a)5 or 16(e) are met. Since the SIP/SOP is connected to functional earth, the reference voltage from mains part to SIP/SOP is the mains voltage.

Figure 3. Insulation diagram for a typical medical product (neglecting AP).


 

POSSIBLE PITFALLS USING ITE POWER SUPPLIES

Knowing which insulation types, reference voltages, and earthing types are required by a product design has physical construction and test consequences. Insulation types are evaluated by examining physical distances (creepage and clearance) and, in some cases, checking insulation thicknesses (transformer layer insulation). Insulation materials provided in lieu of physical air separation are tested for dielectric strength. All distances and insulation thicknesses are evaluated with leakage current measurements. The following section examines each of the insulation requirements in more detail.


Table III. Comparison of creepage, clearance, and dielectric-strength requirements.

Creepage and Clearance. Taking the insulation types and reference voltages determined as necessary in the simple insulation diagram, we can look up the creepage and clearance distance requirements. IEC 60601-1 specifies creepage and clearance requirements in Clause 57.10. Table III shows these distances for both medical equipment (IEC 60601-1) and ITE (IEC 60950). For ITE, we've assumed typical Installation Category II, Pollution Degree 2, and Material Group IIIb. Air-clearance requirements specified by IEC 60950 include a component dependent on maximum repetitive peak voltages, whereas air clearances specified by IEC 60601-1 do not. Repetitive peak voltages are common in switching power supplies. Table III assumes that any repetitive peak voltages do not exceed 420 V. Should repetitive peak voltages exceed 420 V—and this is possible with today's switching power supplies—repetitive peak voltages of 787 V must be present before the clearance requirement of IEC 60950 equals the 5-mm requirement of IEC 60601-1. Repetitive peak voltages higher than 787 V are rare in today's switching power supplies.

As shown in Table III, in all cases creepage and clearance distances required by IEC 60601-1 are greater than those required by IEC 60950. This does not mean that ITE power supplies are unacceptable in all medical devices, but rather they have not been validated as complying with IEC 60601-1. The medical device manufacturer must do this validation.

Figure 4. Creepage and air clearance distances.


 

First, let's review what creepage and clearance distances are. Creepage distance (CR) is the shortest path along the surface of insulating material between two conductive parts (sometimes called over-surface spacing), while air-clearance distance (CL) is the shortest path in air between two conductive parts (sometimes called through-air spacing). Creepage and clearance are illustrated in figure 4.

Figure 5. Circuit blocks.


 

Next, let's review how the circuit blocks of the insulation diagram map to an actual product. Consider the product represented by the schematic in figure 5. This is a product with a mains-connected motor. There is a mains-connected transformer that supplies low-voltage secondary power for control circuitry. The SIP/SIP connection might be for a foot-switch control that activates the solid-state relay and therefore the motor. The product could be, for example, a smoke-evacuation device used to assist with RF surgical procedures.

Notice that there is a single mains part circuit block, a protectively earthed enclosure, and a SIP/SOP circuit block, mapping exactly what is illustrated in figure 3. The primary advantage to insulation diagrams is that they provide a layer of abstraction above the details of the product and allow one to focus on the insulation and earthing requirements.

When considering whether the required basic insulation requirements from mains to earth are met for the smoke-evacuator example shown in figure 5, it's clear we need to consider such components as an appliance inlet, fuse holders, Y capacitors, transformer (core-earthed), motor (enclosure-earthed), printed wiring board, and wire insulation. To verify the required double- or reinforced-insulation requirements from mains to SIP/SOP for the same device, we must consider such components as the transformer, opto-coupler, relay, printed wiring board, and wire insulation.

Figure 6. Printed wiring board example.


 

Figure 6 illustrates exactly what creepage measurements are needed in order to validate that the requirements are met. With the components mounted on the board, clearance distances will need to be measured. If this printed wiring board was designed to meet the minimum requirements of IEC 60950, the required IEC 60601-1 distances may not be provided.

Table IV. Comparison of transformer layer insulation requirements.


 

Transformer Layer Insulation. The transformer layer insulation requirements for medical equipment can be found in IEC 60601-1, Clause 57.9.4e. Table IV shows the transformer layer insulation requirements for medical equipment alongside the solid insulation requirements for ITE (IEC 60950). As can be seen in the table, for reinforced insulation the IEC 60601-1 transformer layer insulation requirements are not represented by IEC 60950 requirements. The medical device manufacturer must validate that the transformer meets IEC 60601-1 requirements and not the minimum requirements of IEC 60950.

Figure 7. Cross section of transformer layer insulation, showing a bobbin with insulating partition.


 

As an example, let's consider two common transformer constructions. The first is a transformer with a center flange bobbin, which is common for linear transformers. A cross-sectional view of such a transformer is shown in figure 7. For a medical device, the thickness of the center flange must be a minimum of 1 mm (single material).

The second construction is a concentrically wound transformer, which is common in switching power supplies; figure 8 shows cross-sectional views from the top and side. We must validate that the insulation material between primary (mains) and secondary (SIP/SIP) windings meet the transformer layer requirements. Because two layers of insulating material will normally be less than 0.3 mm, this typically requires validating that at least three layers of insulating material are provided.

Figure 8. Transformer layer insulation, showing a bobbin with insulating tape.


 

Figure 9. Transformer creepage, showing a bobbin with insulating tape.


The concentrically wound transformer provides an excellent example of an important creepage distance that must be considered. Figure 9 shows where the primary-to-secondary insulation butts up against the bobbin end flanges. This joint is considered an uncemented joint, and the distance between primary and secondary end turns needs to meet the creepage distance requirements, with one special allowance in that the enamel coating on the primary and secondary windings contributes 1 mm each towards the required creepage distance. Shown in figure 9 is a margin tape of width W, which provides positive end-turn retention and maintains the minimum required creepage distance. For IEC 60601-1, the width W will need to be at least 3 mm in order to meet an 8-mm creepage distance requirement after taking into account the 2-mm contribution of the enameled windings. For IEC 60950, W will need to be at least 2.5 mm to meet a 5-mm requirement, since for IEC 60950 the enamel on windings contributes nothing to creepage distances. In addition, creepage distances from winding end turns to opposite winding exit leads must be considered.

Figure 10. Example of a concentrically wound transformer.

 

Figure 10 illustrates a concentrically wound transformer and the type of disassembly and activities that are needed in order to validate that the transformer's construction complies with insulation requirements.

Dielectric Strength. IEC 60601-1, Clause 20 outlines the dielectric-strength requirements for medical equipment. Table III shows these dielectric-strength requirements for medical equipment alongside those for ITE (IEC 60950).

As Table III illustrates, the IEC 60601-1 dielectric-strength requirement of 1.5 kV for basic insulation with a reference voltage of 240 V ac is the same as that in IEC 60950. However, for reinforced insulation, the IEC 60601-1 dielectric strength requirement of 4 kV is larger than the IEC 60950 requirements of 3 kV. The medical device manufacturer must validate that the transformer meets IEC 60601-1 requirements from mains to SIP/SOP and not the minimum requirements of IEC 60950. In order to do this, a test voltage with the same waveform and frequency is applied across the insulation under test. Initially, not more than half the prescribed voltage is applied, then it is gradually raised over a period of 10 seconds to the full value, and maintained for 1 minute. This is illustrated in Figure 11.

Figure 11. Dielectric-strength test. Double or reinforced insulation normally needs to be tested separately.


 

A practical issue with testing reinforced insulation by applying the test voltage on the whole product is that there can also be parallel paths of insulation that require a less-stringent test voltage due to assistance from protective earthing. For example, consider the product illustrated in Figures 3 and 5. If an external test voltage of 4000 V ac is applied from mains to SIP/SOP, there are three paths being stressed, of which only two require reinforced insulation. The first is the mains transformer, which needs the 4000-V-ac test. The second is the solid-state relay, which also needs the 4000-V-ac test. The third is the combination of the Y capacitors—which are required to comply with a test voltage of 1500 V ac (basic insulation for 240 V ac)—and the functional earthing connection between the enclosure and the SIP/SOP circuitry. It is this third path that might reveal a false-negative test result. In order to conduct a dielectric-strength test on only the components requiring the test voltage, it's often necessary to test the components separately from the product. In this example, it is acceptable—and in fact prudent—to remove the mains transformer and solid-state relay from the product and test them separately.

Leakage Current. Leakage current can be defined as all currents, including capacitively coupled currents, that can be conveyed between exposed conductive surfaces and earth or other exposed conductive surfaces.1 Specifically, earth leakage current is current flowing from the mains part through or across insulation into the protective earth supply connection. Enclosure leakage current is current flowing from the enclosure to the operator or patient, with the operator or patient referenced to earth or to another part of the enclosure. One common source of leakage current is EMC filtering components.

Table V. Comparison of leakage current requirements.

 

Leakage current should be kept within acceptable limits for protection against electric shock. Leakage current requirements for medical equipment are specified in IEC 60601-1, Clause 19, where limits are detailed for normal and single-fault conditions. Table V shows the leakage current requirements for medical equipment alongside those for ITE (IEC 60950).

As shown in Table V, IEC 60601-1 leakage current requirements are not represented by IEC 60950 requirements. Because patients can be unconscious, connected to multiple equipment, or have open skin regions (which lowers body impedance), the lower limits in IEC 60601-1 are considered appropriate for medical equipment.

Figure 12. Earth leakage current test circuit.


 

Figure 13. Enclosure leakage current test circuit.


 

Device manufacturers must validate that any medical product employing an ITE power supply meets IEC 60601-1 leakage current requirements. This is accomplished by following the test procedures outlined in Clause 19. The product is connected to a supply mains voltage of 110% of rating. The human body impedance of the operator or patient is simulated with a measuring device (MD) of nominally 1000 ‡ impedance (nominally 2000 ‡, IEC 60950). Leakage current measurements are made under normal and single-fault conditions. Switches are provided in the supply circuit to simulate reverse polarity, S5; a supply conductor opening, S1; and the protective-earthing supply conductor opening, S7. The opening of S1 or S7 is considered a single-fault condition. Only one single fault can be simulated at a time. Figures 12 and 13 illustrate the test circuits for earth leakage and enclosure leakage current measurements, respectively. Required as well—though beyond the scope of this article—are patient leakage current and patient auxiliary leakage current tests.

Summary of Possible Pitfalls. As the preceding review shows, if an ITE power supply is being relied on to provide basic insulation from mains to an earthed enclosure and/or double or reinforced insulation from mains to SIP/SOP, an evaluation must be done to determine whether the ITE power supply complies with IEC 60601-1 requirements. This evaluation must include:

An examination of creepage and clearance.
An examination of transformer layer insulation.
Testing of dielectric strength.
Testing of leakage current.
There are other requirements that IEC 60601-1 mandates but IEC 60950 does not. These requirements include, but are not limited to, the following:

Two fuses for Class I equipment (Clause 57.6).
In some cases, basic opposite polarity distances (20.2 A—f, 57.10.b).
In some cases, additional transformer construction and abnormal testing (57.9).
Humidity conditioning (4.10, 44.5).
As indicated in IEC 60601-1, Appendix A, the rationale for why many of the requirements for protection against electric shock are more stringent in IEC 60601-1 than in IEC 60950 include:

Absence of normal reactions in a patient who may be ill, unconscious, anesthetized, immobilized, or incapacitated in some other way.
Absence of normal protection to current provided by the patient's skin, if the skin is penetrated or treated to obtain a low skin resistance.
The simultaneous connection of the patient to more than one piece of equipment.
The application of electrical circuits directly to the human body, either through contacts to the skin and/or through the insertion of probes into internal organs.
OPTIONS FOR OVERCOMING SOME INCOMPATABILITIES

Isolating Transformer, Earthed Secondary. Many medical products employ an isolation transformer in the mains circuit, in which case the secondary winding then supplies all internal circuitry for the product. Let's consider first an isolation transformer with the secondary tied back to protective earth. The ITE power supply is supplied from the isolated, earthed secondary winding. We'll assume a secondary voltage of 120 V ac, and that the ITE power supply has been evaluated to IEC 60950 for 240 V ac.

Figure 14. Isolated secondary tied to protective earth.

 

Figure 14 shows an insulation diagram for such a product. Since the 120-V secondary is earth referenced, the ITE power supply needs insulation much as if it was located in the mains, except that now the reference voltage is 120 V ac. The power supply will need to be checked for compliance with creepage requirements.

Table IV shows the transformer layer insulation requirements. In the case of single or double layers, the transformer will need to be checked for compliance. In the case of three layers, since the reference voltage is 120 V, a 3000-V test is required on two layers, and since this is the same requirement as in IEC 60950, no further evaluation will be needed.

As Table III shows, the dielectric-strength requirements are the same in both standards; so no further evaluation is needed.

The leakage-current requirements are represented in Table V. Leakage-current testing will be needed, although the earth and enclosure leakage is likely to be low since the power supply's leakage current is referenced to the secondary of the isolation transformer.

To summarize, the power supply must be checked for compliance with the IEC 60601-1 requirements for creepage, transformer layer insulation, and leakage current.

Isolating Transformer, Floating Secondary. Let's consider the same product, except this time with the isolated secondary floating from earth. Figure 15 shows an insulation diagram for such a product. Because the 120-V secondary is floating, both sides of the 120-V circuit must be accessible for it to be an electrical shock hazard. Providing basic insulation for 120 V from the secondary to the enclosure and supplementary insulation for 120 V from the secondary to the SIP/SOP creates a situation in which two faults would be needed in order to have both sides of the 120-V secondary accessible.

This protection scheme provides a good opportunity to explain why it may be important to have the occurrence of a fault detectable by the user. In this example, the failure of either the basic or supplementary insulation may not be obvious to the user. The safety of the product then relies on a single level of protection, and is therefore diminished. An isolation monitor with an alarm or a periodic maintenance program that checks for changes in leakage current are examples of fault-detection techniques that could be appropriate for a product employing this design. This is not something required by IEC 60601-1, but might be considered appropriate as a result of a manufacturer's risk analysis.

A comparison of creepage and clearance requirements in IEC 60601-1 and IEC 60950 is shown in Table III. Creepage distances to chassis will also need to be checked. Because the transformer is not relied on for reinforced insulation, there are no transformer layer requirements (Table IV).

Table III also shows the dielectric-strength requirements, for which no further evaluation is needed. Testing for leakage will be needed, but because the isolated secondary is floating, the leakage currents are expected to be low (Table V).

Thus, for this configuration, the power supply only must be checked for compliance with IEC 60601-1 requirements for creepage and leakage current.

Figure 15. Isolated secondary floating from protective earth.



 

Figure 16. Data-separation PWB.

 

Data-Separation Device. Let's consider the ITE power supply in the 240-V-ac mains. This time, we'll rely on the mains isolation only for supplementary isolation and provide a basic insulation barrier between the ITE power supply outputs and the SIP/SOP circuits. An insulation diagram for such a design is shown in Figure 16.

As Table III shows, the creepage and clearance distances to the chassis will need to be checked. Once again, because the transformer is not relied on for reinforced insulation, there are no transformer layer requirements (Table V). No further evaluation is needed of the dielectric-strength requirements (Table III). The leakage requirements (Table V) will need to be tested; modifications to the input EMC filter stage might overcome any problems encountered.

Thus, for the data-separation configuration, the power supply only must be checked for compliance with the IEC 60601-1 requirements for creepage and clearance and leakage current.

CONCLUSION

Using ITE power supplies in medical equipment requires additional measurements and testing that may reveal areas of noncompliance. By specifying IEC 60601-1 power supplies, manufacturers can greatly facilitate validating that their medical equipment complies with IEC 60601-1.

Ideally, the following should be included in the power supply specification:

Compliance with Medical Electrical Equipment—Part 1 General Requirements for Safety, IEC 60601-1, 2nd edition, (1988-12) + Amendment 1 (1991-11) + Amendment 2 (1995-03) + national/regional deviations as needed.
Environment conditions specified in IEC 60601-1, Clause 10.2.1, or higher, if needed.
A ±10% supply mains tolerance.
Electrical ratings.
Insulation type and nominal reference voltage needed from input to output(s) (only the vendor will know the specific reference voltage, based on design).
Insulation type and nominal reference voltage needed from input to chassis.
Any third-party certification marks and/or test reports required.
 

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 Power Transformer Information:

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• Federal Pacific Transformers -  Federal Pacific is a division of Electro- Mechanical Corporation, a privately held, American owned company founded in 1958. Federal pacific offers dry-type transformers from .050 KVA through 10,000 KVA single and three phase, up to 34.5 KV, 150 KV BIL with UL approval through 15 KV.
• Marcus Transformer - Ever since they opened their doors for business a half a century ago, they have been a leader in innovative transformer design. As a family-owned company they are proud of the reputation they have earned for making quality-built transformers that deliver exceptional performance and savings.
• Hammond Transformers -  Hammond Manufacturing was founded in 1917 in Guelph, Ontario, Canada. In the last 3 decades it has expanded to the US and the international markets offering many types of power transformers. 
• TEMCo Transformers - TEMCo Transformer, a family-owned business which has been manufacturing and distributing electrical products since 1968. They focus on transformers that significantly reduce power consumption over 30 percent compared to competitive makes.
• GE Transformers - GE has been a key player in the energy industry for more than a century.  Since the installation of their first steam turbine in 1901. They have become number one provider of high-technology power generation and distribution equipment.
• Jefferson Electric Transformers - Jefferson Electric has been a pioneer and innovator of magnetic products since 1915. Jefferson broad line of dry-type transformers are backed by quality assurance systems so stringent that each and every unit gets thoroughly tested before it goes out there door.
• More power transformer brands - Check out more companies by clinking this link.

 Power Transformer Types

• Distribution Transformers - Distribution transformers are generally used in electrical power distribution and transmission power. This class of transformer has the highest power, or volt-ampere ratings. and the highest continuous  voltage rating.
• Substation Transformers - Substation Transformers are large devices which usually weigh tens of thousands of pounds.   They are filled with tens of thousands of gallons of heat transfer fluid.  Although they are typically 99.8% efficient in the transforming of electricity from one voltage to another, processing hundreds of Mega Volts-Amps of electricity force the liberation of hundreds of BTUs per second.
• Medical Grade Isolation
   Transformer - Medical Grade Transformers generally refer to the transformers used in medical devices as well as hospital, biomedical and patient care equipment. There are a number of strict safety rules, guidelines and laws governing the design, construction and the test of these transformers.
• Drive Isolation Transformer - They are used to isolate a drive from a main power line to prevent the transmission of harmonics that the drives produce back into the power line.  They stop drive harmonics from disrupting computers and other sensitive equipment.
• Toroidal Transformers - Toroidal Transformers are more efficient than the cheaper laminated EI types of similar power level. Some of the advantages are smaller size, lower weight, less mechanical hum, (making them superior in audio amplifier), low-off-load loss.

 Power Transformer Types

• Step-Up Transformers - A Step-Up Transformer is one whose secondary voltage is greater than its primary voltage.  This kind of transformer "steps up" the voltage applied to it. -
• Step-Down Transformers - A Step-Down Transformer is  designed to reduce voltage from primary to secondary.  They can range from sizes from .05 KVA to 500 KVA
• Isolation Transformers - An Isolation Transformer is a device that transfers energy from the alternating current (AC) supply to an electrical or electronic load.  It isolates the windings to prevent transmitting certain types of harmonics.
• Buck Boost Transformers - Buck Boost Transformers make small adjustments to the incoming voltage. They are often used to change voltage from 208v to 240v for lighting applications.  Major advantages of Buck boost transformers include; low cost, compact size and light weight. 
• High Voltage Transformer - There are many different types of voltage transformers. A High Voltage Transformer operates with high voltages. Typically, these voltage transformers are used in power transmission applications, where voltages are high enough to present a safety hazard.
• Medium Voltage Transformers - A Medium Voltage Transformer can be connected directly to a primary distribution circuit and generally has the most load diversity. These voltage transformers have installation practices that are generally in accordance with application recommendations from the Institute of Electrical and Electronic Engineers (IEEE).
• Low Voltage Transformers - A Low Voltage Transformer is an electrical device that transforms 120 volts (line voltage) into 12 volts or 24 volts (low voltage). Some uses for low voltage transformer are in landscaping lighting.
• Single Phase Transformers - In electrical engineering, single-phase electric power refers to the distribution of electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors.
• Three Phase Transformers - Three Phase Transformers must have 3 coils or windings connected in the proper sequence in order to match the incoming power and therefore transform the power company voltage to the level of voltage needed while maintaining the proper phasing or polarity.
• Custom Transformers - Custom Transformers are designed for a certain performance specifications and size requirements.  The company works with your engineering specification. 
• Industrial Control Transformers - Industrial Control Transformers are used to convert the available supply voltage to the required voltage to supply industrial control circuits and motor control loads.
• Pad Mounted Transformers - Pad Mounted Transformers are usually single phase, or three phase, and used where safety is a main concern. Typical applications; restaurant, commercial building, shopping mall, institutional. 
• Pole Mounted Transformers - Pole Mounted Transformers are used for distribution in areas with overhead primary lines. Outside a typical house one can see one of these devices mounted on the top of an electrical pole.
• Oil Filled Transformers - Oil Filled Transformers are transformers that use insulating oil as insulating materials.  The oil helps cool the transformer. Because it also provides part of the electrical insulation between internal live parts, transformer oil must remain stable at high temperatures over an extended period.
• Dry Type Transformers - Dry-Type Transformers are available for voltages up through 34.5 kV (although the most common upper limit is 15) and KVA ratings up through 10,000 (with 5000 as the usual limit). Dry-type use air as a coolant, lowering health and environmentally concerns.
• Auto Transformers - An Autotransformer is an electrical transformer with only one winding. The winding has at least three electrical connection points called taps. Autotransformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 138 kV to 66 kV for transmission. Another application is in industry to adapt machinery built for 480 V supplies to operate on the local 600 V supply.
• More power transformer types - Read further about additional transformer types and their uses.

 Power Transformer Term Definitions

• Electrical Transformers - Electrical Transformers are devices used to raise or lower the voltage of alternating current. For instance, power is transported over long distance in high voltage power lines and then transformers lower the voltage so that the power can be used by a business or household.
• Isolating Transformers - An Isolating Transformer is a transformer, often with symmetrical windings, which is used to decouple two circuits.  An Isolation transformer allows an AC signal or power to be taken from one device and fed into another without electrically connecting the two circuits. Isolation transformers block transmission of DC signals from one circuit to the other, but allow AC signals to pass. 
• Transmission Power Lines - A Transmission Line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electromagnetic or acoustic waves as well as electric power transmission. Components of transmission lines include wires, coaxial cables,  dielectric slabs, option fibers, electric power lines, and waveguides.
• Transformer Voltage - The measure of the amount of force on a unit charge because of the surrounding charge.
• Transformer Phase - Most transformer are either single phase or three phase.
• Transformer Frequency - The transformer cannot change the frequency of the supply. If the supply is 60 hertz, the output will also be 60 hertz.
• Transformer K Factor - Some transformers are now being offered with a k-factor rating. This measure the transformer's ability to withstand the heating effects of non-sinusoidal harmonic currents produced by much of today's electronic equipment and certain electrical equipment.
• Primary Voltage - The coil winding that is directly connected to the input power.
• Secondary Voltage - The coil winding  supplying the output voltage.
• Harmonic Cancellation - Harmonic cancellation is performed with harmonic canceling transformers also known as phase-shifting transformers. A harmonic canceling transformer is a relatively new power quality product for mitigating harmonic problems in electrical distribution systems. This type of transformer has patented built-in electromagnetic technology designed to remove high neutral current and the most harmful harmonics from the 3rd through 21st.
• Weatherproof - Enclosed transformers come with a weatherproof standard set by NEMA.
• Epoxy Encapsulated - A process in which a transformer or one of its components is completely sealed with epoxy or a similar material. This process is normally preferred when a unit might encounter harsh environmental conditions.
• More power transformer terms - Such as inductor, ground fault, core saturation, current transformer, faraday shield, etc.

Related Transformer Products

• Voltage Regulators - A Voltage Regulator is an electrical regulator designed to automatically maintain a constant voltage level.  It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.
• AC Line Reactor - AC Line Reactors is a three phase transformer used in conjunction with AC variable frequency and DC motor drive. They are a bi-directional protective filtering device.
• Line Power Conditioners - Power or Line Conditioners regulate, filter, and suppress noise in AC power for sensitive computer and other solid state equipment.
• DC Power Supplies - Conversion of one form of electrical power to another desired form and voltage. This typically involves converting 120 or 240 volt AC supplied by a utility company to a well-regulated lower voltage DC for electronic devices.
• Rotary Phase Converters - Rotary Phase Converters are commonly used in home or small commercial or industrial settings. Rotary phase converters convert single-phase power into three-phase power. This is a very cost-effective way to power three-phase electric motors and other three phase equipment.
• Frequency Converters - A Frequency Changer or Frequency Converter is an electronic device that converts alternating current (AC) of one frequency to alternating current of another frequency.
• Voltage Converters - A Voltage Converter changes the voltage of an electrical power source and is usually combined with other components to create a power supply.
• Magnetic Motor Starters - Magnetic Motor Starters are essentially heavy duty relays mounted in boxes, often equipped with heater/thermal overloads matched to the motor they start.
• Motor Starting Auto Transformers - An Auto Transformer starter uses an auto transformer to reduce the voltage applied to a motor during start. The auto transformer may have a number of output taps and be set-up to provide a single stage starter, or a multistage starter.

For an additional resource the Best of Industry Web Directory : Electrical Power Transformer Directory section is quite useful.