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EARTHBONDING TESTS (Rigel Medical Practical Guide to IEC 62353)

High Test Currents (10A or more) might potentially (my itallics) be destructive to parts of the unit under test which are connected to the protective Earth but have a functional purpose (e.g. screening). As such, consideration should be given to the test current.

Low Test Currents (<8A) could potentially influence the reading as contact resistance is influenced by a number of factors (Constriction, Pressure, Film resistance).
Higher Test Currents overcome the contact resistance where lower currents show a relatively higher reading, thus potentially causing unnecessary failures.

We still have a lot of kit in Theatres, such as stack systems with earthed chassis isolating transformers on long mains leads with multiple bonding connections which require the use of 10A and higher earth bonding tests. It would appear that deciding to go down the line of exclusive use of IEC 62353 would give rise to an increased use of Medical system PE failures (after the checking of individual devices).

Whilst we are considering the differences between the two standards, the IEC 60101 checks earth leakage in Single and Double Fault condition, I am unable to determine if the IEC 62353 performs the earth leakage test in Double Fault condition?

IEC 62353 only specifies a minimum earth bonding test current (>200mA either AC or DC) and leaves the choice up to the user. Most electricians have been using a 200mA DC test current for many years now and they have no problem with using this lower current.

In most cases a test instrument designed to conduct IEC 60601 tests can be either upgraded by software to conduct IEC 62353 tests or be adapted to conduct the direct method tests, by simple re-configuration of connections to the DUT.

As test equipment must inevitably be replaced it is prudent of any organisation to purchase new equipment that complies with the latest standards, particularly if those standards reduce test time and possibly increase safety.

A formal standard for testing medical equipment can only be a good thing as it removes the guesswork for those who may not have the experience of knowledge to decide what tests to select from the IEC 60601-1 type test standard.

I am not trying to encourage anyone to rush out and replace all their existing equipment with new IEC 62353 testers but there seems to be quite a few who do not want to even entertain the possibility that 62353 is the way forward and to investigate it properly and toughly.

I have seen too many Biomeds simply connect a device to a tester, press the button and wait for a pass/fail message. They do not look at the results to see if they happen to be close to the limit or even have increased since the last test and so may well miss an imminent failure. At least IEC 62353 formalises the need to compare previous values which must be a good thing?

Large EBME departments with many experienced engineers may become a thing of the past as time goes on and cutbacks prevail. It is essential to have a formal test standard that can be easily adopted by smaller groups and make enforcement by the HSE simpler than to try to have the current free-for-all situation.

The IEC 62353 direct equipment leakage check is only tested in a single fault condition but with mains polarity in the normal and reversed conditions.

If you want to perform the double fault condition test with an open earth and open neutral you can use the IEC 62353 alternative method equipment leakage test. This will provide a result exactly the same as the open earth/open neutral IEC 60601-1 test, assuming the mains supply is 230V + 10% (as the tester uses a 253V AC source).

It is really worth while getting to know IEC 62353 as it is not as bad as some would have you believe!

Hi Sean,
Double fault condition is not part of any design standard (like 60601) or routine test standard like IEC 62353. 1 fault is allowed, 2 is not. IEC 62353 tests in single fault condition only accept for differential method. Providing a double fault condition would mean you require to determine your own pass / fail limits as none of the medical standards provide this.

As for test currents,IEC 62353 is clear in that it states minimum of 200mA and that you are free to use 10A if you want to.

Hope this helps

john

If you had a very thin piece of wire making earth continuity e.g. an earth wire had come adrift and one strand was still making contact with the terminal, then at low currents this would pass a safety test. But if in the real world situation the live was to touch the chassis this strand would blow like a fuse and the case would become live.
Why are low current tests allowed?
I know this is a double fault condition but the low current earth bond test is not showing up the first fault.
RoJo

Hi rojo
Have you tried this yourself? I bet a low current test will show up a high resistance value for a single strand as
A) the mechanical contact will have been significantly compromised if this was the case (ie providing constriction resistance) and
B) the cross section of the earth wire will have been significantly reduced thus increasing the resistance for electrical current.

I have done tests whereby the earth screw was loosened (could happen due to vibration and more likely than the single strand scenario), compromising both the mechanical and electical security of the protective earth (ie high constriction resistance for electrical current). A 25A tests gave a good pass, the low current saw a high resistance and failed. Just another scenario to consider.............

JB I have not done this it was just a thought experiment and to add to the general thought of the discussion.
RoJo

Both arguments are valid, in my opinion.

Yet so is the approach of looking at things from the oft-mentioned "real world" perspective:-

Most detachable (that is, almost all) BS-1363 (UK 13A) to C13 (IEC) mains cables are rated at 10 Amps. So 10 Amps would seem to be a realistic compromise at which to test.

Just like the Goldilocks Principle:- not too much, not too little, but just about right!

But we're talking here about electrical safety. It is a fundamental point that the protective earth conductor has to be man enough to carry any fault current long enough to blow the equipment fuse(s). That is, the fuse(s) must blow, not the earth conductor!

IEC-60601 talks about an AC current (yes, at 50 Hz) of between 10 and 25 Amps. Then HE-I95 wimped-out to only 1 Amp. What is it now, 200 mA? So much, then, for progress!

The way that the protective earth ultimately works is that a live touches the case which cause a low impedance hence high current path to earth which blows the plug or equipment input fuse. Ultimately the earth path must pass sufficient current to allow the fuse to blow before it does. How do you test that? Use a large enough current in the earth testing path?
RoJo

Hi Rojo,
before the fuse goes, the RCD should trip at leakage over 30mA. Do you suggest we test at 30mA then?

Progress is all about doing more with less, unless you talk to the Tories ;-)
John