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Leakage Current Testing Requirements for Medical Grade Transformers

While all transformer applications must meet certain specifications, the guidelines for medical grade transformers are especially strict, as their performance plays a direct role in patient health and safety.

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 applications than others. Leakage in medical transformers, in particular, can have serious consequences if these currents flow through a patient: this can be directly deadly (as in electric shock) or can interfere with other monitoring devices such as EKG or other equipment. Therefore, original equipment manufacturers (OEMs) operating in the medical equipment industry must be sure to thoroughly consider transformer design and leakage current measurements.

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. 

UL is the official regulatory body for the United States, 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, testing, and certification body in Europe, working closely with each nation’s own national laboratory.

The UL 60601-1 standard, which replaced the original UL 544 standard, cites 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.

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” currents and is not acceptable for measuring patient leakage currents.

Modern standards for leakage current require that the transformer be properly energized and then to measure the leakage current.  Since today’s insulating materials are extremely good, 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).  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 misspecified; 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 product malfunction or outright failure. 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 secure the UL and IEC ratings your equipment is required to carry if it is to be sold; this applies in virtually every market. 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 secure a certification from UL or IEC 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 Grade Transformers from Triad Magnetics

For over 60 years, Triad Magnetics has been a leading manufacturer of both standard and custom magnetic components.

We’ve developed an extensive base of knowledge and expertise in the design and manufacture of transformers, including medical grade transformers, which we design and build specifically to minimize the leakage currents that jeopardize proper equipment functioning. With state-of-the-art production facilities and thorough, cutting-edge testing and diagnosis capabilities, we’re proud to offer our clients reliable services, every time.

To learn more about Triad Magnetics and our custom medical transformer manufacturing capabilities, or to discuss our product inventory — such as our medical toroidal transformers — contact us today.

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