Leakage Current Testing Requirements for Medical Grade Transformers

While all transformer applications must meet certain specifications, the industry safety standards for medical grade transformers are especially strict, as their performance plays a direct role in patient health and safety.  The same can be said in terms of occupational safety for the doctors, nurses and technicians who interact with the wide range of electronic equipment that supports patient diagnostics, treatment and facility operations (Fig 1).

Leakage current, in particular, is one of the most important specifications for medical grade transformers. This refers to any current that passes through the dielectric insulation. This can be current that physically “leaks” through the insulation, or capacitive currents that appear to cross even exceptionally good insulation.

Although leakage current is never desirable, it’s a much more serious concern for some medical applications than others. Leakage in medical transformers, in particular, can have serious consequences if these currents flow through a patient or medical staff: this can be deadly (as in electric shock) or can interfere with other monitoring devices such as EKG or other equipment.     

To assure patient safety, original equipment manufacturers (OEMs) operating in the medical equipment industry must be sure to consider thoroughly the impact of transformer design and the measurement of potential current leakage.  It is important, therefore, to understand the industry’s safety standard approval requirements, how that impacts their design and the other benefits of medical grade transformers.

International Leakage Current Standards

Today, the International Electrotechnical Commission (IEC) and Underwriters Laboratories (UL) are the two main regulatory bodies that determine and publish minimum safety standards for electronics products, including medical transformers and current leakage.

UL is the official regulatory body for the United States (U.S.), as it was appointed by the Occupational Safety and Health Administration (OSHA) to both test and certify all electronic equipment. The IEC is the standards body in Europe, which works closely with each nation’s own national laboratory. For example, ANSI/AAMI ES 60601-1 is a standard that has been harmonized with IEC 60601-1 to help ensure global electrical safety is consistent from nation to nation.

The ANSI/AAMI ES 60601-1 standard specifies the maximum allowable leakage current values, which differ depending on equipment class and whether the equipment is located in a patient care area, such as an exam, overnight, or operating room.  The largest allowable leakage current is 500 microamps (µA) for Class I non-patient care area equipment; as the equipment classes progress, this number steadily decreases. IEC 60601 follows very similar guidelines.

Please note that these standards specify the performance of the completed medical device; they do not specify limitations of the transformer. However, having a low leakage transformer can greatly simplify the ease in which a completed device will meet leakage requirements.

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Figure 2. Earth Leakage Testing Set-Up

Measuring Leakage Current

In the past, leakage current was measured by applying the maximum expected input voltage between the primary(s) and secondary(s) at the power line frequency and measuring the current that flows capacitively between the windings, assuming the return path was through ground. This is still tested today, but is considered “earth leakage” current (Fig 2) and is not acceptable for measuring patient leakage currents.

Modern standards for “patient leakage” current (Fig 3) require that the transformer be properly energized before measuring leakage current. Today’s insulating materials are extremely good, with the result being that most leakage current is due to the capacitive currents that are created from the difference in voltage across the insulating barrier. For example, 120V line voltage has one lead with a somewhat large voltage (the “hot” lead) and the other lead has very little voltage compared to ground (the “neutral” lead). 

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Figure 3. Patient Leakage Testing Set-Up

More capacitive currents will flow from the higher voltage of the hot end of the primary winding then will flow from the lower voltage of the neutral end. The modern standards for patient leakage current tests now take this into consideration, where the older standards did not.

The Importance of Testing

Using established best practices to test leakage current of transformers — whether medical grade or otherwise — is beneficial in several ways. First, testing for leakage current through reliable methods allows for more accurate specifications of transformers and the products in which they’re used. Transformers that have been tested poorly can easily be mis-specified; for example, using incorrect or incorrectly calibrated testing equipment could show leakage current of a transformer at 100 µA, when in reality it is closer to 300, 400, or even 500 µA.

Using such a transformer in a piece of equipment could lead to a system not meeting the leakage current requirements. If you purchase transformers that have been tested properly, however, you can rest assured knowing that a 100 µA leakage current transformer will reliably perform with leakage current of 100 µA or less.

More importantly, using well-tested transformers in your medical equipment will help guarantee regulatory compliance for your equipment. A great deal of time, energy, and money goes into designing medical equipment, and transformers are often crucial elements; ensuring their safety and reliability is paramount. If a given piece of equipment fails to comply with regulatory standards because of excessive transformer-related leakage current, it will essentially have to be redesigned from the ground up — an extremely time-consuming, costly endeavor.

It is important to ensure that transformers are truly specified for patient leakage current and not just earth leakage current.  Many specifications conveniently lack this clarification, showing a typically low earth leakage current and hoping that this will be interpreted as a true patient leakage current.

Medical Vs Standard Transformers

It is important to know the differences and advantages of medical grade transformers versus standard grade transformers.  Medical transformers are almost exclusively used as isolation devices to protect the patient and medical staff from the electronic equipment used in patient care and medical facilities.

A safety shield can be placed between the primary and the secondary coils, with insulation serving as the isolation layer between each coil, which keeps the device within the maximum level allowed for current leakage.  If the isolation fails, the AC input current is immediately redirected to ground through the shield, which is designed to trip the over-current device.

Double insulation is designed into medical grade transformers.  These devices are designed with two or more layers of insulation.  Should one layer of insulation fail, the other(s) will maintain isolation to minimize current leakage.

There are a number of benefits beyond safety to the use of medical grade transformers, which include:

·       UL/IEC approved transformers reduce safety testing time and speed time-to-market for medical equipment.

·       They inherently feature a low leakage current design.

·       They offer guaranteed compliance with internationally recognized electrical safety standards.

·       They feature built-in thermal protection for greater resistance to less than ideal operating environments.

The design and performance of medical grade transformers must comply with the harmonized IEC 60601-1 standard, which involves more than simple current leakage.  Their required testing sets maximums on earth, patient or enclosure current leakage.  They must meet the minimum IEC standard for creepage distance and air clearance as well, and they must not exceed the maximum temperature rise.

What To Look For

When specifying a medical grade transformer, there are a number of design and construction features that are important to consider beyond meeting the UL/IEC 6060-1 standard.  While there are any number of approved medical transformers available from multiple manufacturers on the market, Triad’s MD Series (Fig 4) offers a number of important design features that will help medical equipment designers find the best device for their unique application.

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Figure 4. Triad MD Series Medical Transformer

The MD Series comes in eight different two- or four-receptacle models.  They are designed with a toroidal isolation transformer to match the power requirements of a wide variety of equipment. Typical application settings include diagnostic imaging, outpatient care, hospital, dental, laboratory and industrial plants. Key features include:

·       Power range: 250 to 2,400 VA

·       Input Voltage (power cord): 120 or 240 VAC ±10%, 50/60 Hz

·       Output Voltage (receptacle): 120 VAC

·       Voltage Regulation: up to 5% typical

·       Hipot test: input to output: 3000 VAC

·       Operating temperature: 60C max

·       Earth leakage current:  typical 10 uA

Triad’s advanced design creates an “isolated” output voltage that is not referenced to ground. The isolation of the transformer improves safety due to separation from ground, and the fusing and power limitations inherent in the transformer improves the safety of being powered by line energy.

Triad’s MD Series employs a compact toroidal transformer as compared to the hefty El type transformers found in other devices.  Triad’s design and construction helps reduce stray fields, increases power efficiency and minimizes size and weight, as well as providing quieter operation with a noticeably less audible buzz sound. 

This medical isolation transformer is designed and constructed within Class F insulation (155° C) for high performance.  Potential applications include hospital beds, dental chairs and equipment, medical lab equipment, and patient monitoring equipment.

Conclusions

When health and personal wellbeing is at stake, as it often is in medical industry, it is of the utmost importance that equipment functions properly. Medical equipment designed with or supported by approved UL/IEC 60601-1 medical grade isolation transformers is essential for safety and reliability.  

When choosing a medical grade transformer, also be sure to look for more than safety approvals.  The design, construction, materials, quality and testing procedures of the transformer manufacturer can vary significantly and affect performance, reliability and transformer life.

Triad Magnetics