EBME Forums Forums Maintenance Technical Assistance Electrical Safety Testing of Combined Equipment

Hi

Could anyone please tell me what the allowable leakage currents are for combined medical equipment i.e. a medical stack system?
Our practice is to test such items as a single unit, since they are used by staff in this way. Am I right in assuming that under such circumstances the allowed limits can exceed 0.5mA?

I've searched on the internet, but can't find any information to support this. Any assistance would be appreciated.

Thanks

This question has come up from time to time in the past.

Hello SSP

In summary then.....

With a stack system the combined leakage currents in quiescent mode (everything switched on, but not in operation) can be found to exceed IEC601 limits, > 500 uA. (Even though each individual unit may be considerably less than 500 uA leakage current drawn). But, it is the combination of individual leakage currents which presents the danger. That is why we incorporate isolation transformers to reduce the leakage currents to an acceptable level.

High leakage currents occur due to power supply smoothing, and a mixture of medical grade and non medical grade equipment found on a typical stack system.

Possibly what you are referring to is the earth bonding resistance, which can exceed 200 m Ohms in total because of the power distribution arrangements, (<100 m Ohm per leg allowable).
So, mains cable to transformer, transformer to distribution block, distribution block to inlet receptacle.

Having just wasted a bit of time wading through that earlier thread I pointed to (and wishing now that I hadn't done that) ... my short answer to your question would be:- no, it is not permissible. 500 µA is the limit (or 1 mA under Single Fault Condition - but let's not go there, unless you really want to - see all the verbiage below).

But Sean has already provided the "answer":- the famous (but often overlooked) isolation transformer. There have been many previous discussions on here about those as well.

How about this one as "food for thought"?

Myself, whilst respecting all official standards and documents (naturally), I like to consider this stuff from the "actual risk" point of view. That is why I like to measure and record what we might call "risk current". That is, with ground open (protective earth removed) - a possible "real world" fault (known as a "single-fault condition", or SFC). This is properly known as "enclosure leakage current under SFC" (or "touch current" if you like North American clarity - which I do). Current through a 1 K load between chassis and earth* must be less than 500 µA (that's < 500 mV across 1 K). So once again we are back to the magic figure of 500 µA. Note that this test ignores the trailing mains cable (which is out of the test circuit), which to my mind is yet another of its redeeming features!

In a strictly practical sense, only two basic electrical safety tests are actually necessary to ensure that a unit (system, or individual piece of kit) is safe:-

1) Grounding resistance
2) Risk current

But if you only have the time (or the inclination) and indeed the test equipment to carry out just a single electrical safety test, then I would argue that Risk Current is the one to go for. To my mind, it proves that the equipment itself (or in this case, the system) is inherently electrically safe because, when you stop and think about it, it also assures that other tests would "pass" as well (eg, insulation). However, some of the official guidelines do not include this test as such - but what do they know?

Having said that, myself I would also always take the time to physically examine the trailing mains cable, the plug and fuse as well as protective earth continuity. Even if it was only with my trusty multimeter. Because, as we all know, the integrity of the grounding conductor is the essential element of equipment electrical safety. After all, it is the drain down which all accumulations of (esoteric) leakage currents ... er, leak away harmlessly to "earth" (wherever that is)!

* Meaning the best real (practical) earth available near by - usually the earth socket at the nearest mains power outlet.

BS EN 62353 says:

Each individual equipment of an ME SYSTEM which has its own connection to SUPPLY MAINS, or which can be connected/disconnected from SUPPLY MAINS without the use of a tool, shall be tested individually. Additionally the ME SYSTEM as a whole shall be tested to avoid a situation where ageing of individual equipment can result in unacceptable values.

An ME SYSTEM that is connected with a MULTIPLE SOCKET-OUTLET to SUPPLY MAINS shall be treated during the tests like a single item of equipment.

If the ME SYSTEM, or a part of it, is connected to SUPPLY MAINS via a separating transformer, the transformer has to be included in the measurements.

So each item should be less than 500uA as should the total of the stack. If you made the stack up yourself without reference to section 16 in BS EN 60601-1 then you may have a problem!

Section 16.5 When FUNCTIONAL CONNECTION between ME EQUIPMENT and other items of equipment of an ME SYSTEM or other systems can cause the allowable values of LEAKAGE CURRENT to be exceeded, then safety measures incorporating a SEPARATION DEVICE shall be applied.

If you don't have a copy of either document I suggest you obtain one as soon as possible!

Assuming that you have encountered a stack system with excessive combined levels of leakage current, obviously what needs to be done now is to ascertain which element in the system causes the high leakage!

Be sure to let us know how you get on.

Thank you all for your assistance!

Thank you