EBME Forums Forums Maintenance Technical Assistance Acceptance testing

Mark, we use an in line 'box' which measures both earth continuity of the mains lead and leakage currents from the machine on test.
I guess that the weakness of our system is that we cannot really ensure that any connected earth return lead within the equipment on test is operating as intended.
As Richard suggests, an internal inspection and perhaps a megger tester could allay any concerns that you may have but from a practcal point of view perhaps the equipment could be considered as having class 2 protection anyway as there are no accessable parts which could become live to the user and there is a distinct air gap between live parts and an otherwise insulated case assembly.
A can of worms maybe, but we have to consider what risks are present in real terms.
Perhaps these sort of questions should really be directed towards the manufacturers and the regulatory bodies......?

A megger tester doesn't come into it - a Class 1 test (~500Vdc insulation test) is provided on safety testers, pre-programmed - L/N-E, non-earthed accessible parts, i.e. enclosure-earth, applied parts-earth (for BF/CF), for example. A device without an accessible earth can never be assumed to have Class 2 protection - it would be marked as Class 2 if it did.

I'll say it again (just in case it was missed the first and second time) - assume a device with non-accessible earthed parts is Class 2 when it's Class 1 and apply Class 2 EST rather than Class 1 EST using a standard pre-programmed safety tester (in auto or manual mode) then you're likely to miss important tests that establish safety of the device.

In particular if the Class 1 device has either non-earthed accessible, conductive, parts, applied BF/CF parts or both conductive accessible parts and floating applied parts. Safety testers will usually only apply relevant tests depending upon the class and the type of applied parts we tell it we're testing to.

We have to do the rest like choose the correct class, type, connect the DUT and probe test points appropriately, as and when necessary during the test sequence. If we don't then what's the point of testing - it's just wasted time otherwise - and not worth doing at all if it's considered as wasted time when done properly, in my opinion.

Why all this jumping through hoops? If electrical safety testing of medical equipment is required at acceptance and/or during routine maintenance, then guidance (why not regulations, I wonder?) from the appropriate bodies (MHRA or whomever) should be crystal clear. Such guidance needs to be reviewed from time to time in light of "advances" in technology and manufacturing methods, I might add. It should not be for mere mortals like us techs to be for ever wringing our hands over this stuff.

Hopefully there will be guidance and standards on routine testing in the near future; for those that want to be told where to connect the probe, which button to press, how long to press it for, and which indicator to look out for.

I think you already know where you should stick your probe, Richard!

I understand Criticare are retrospectively going to fit an earth stud to the outside casing. Funny that.

Most ClassI equipment has some ClassII parts on it - not every single part of a ClassI device is earthed. These instances are covered in the standards for testing.
In these cases we are dealing with a special case where the whole of the case is a ClassII part of a ClassI device.
If you are testing for electrical safety - not type testing!! - you need to test what you have. Test all the ClassII parts as ClassII parts, if they pass the device will be elctrically safe. Enclosure leakage test being done as per standard with a defined sized foil being applied with a defined pressure, as you do on all ClassII parts of a ClassI device don't you?
How many people use the earth connector to test for enclosure leakage and ignore all the plastic case where you could have inductive and capacitive leakage currents?
The safety of the device is not reliant on it having an earth if there are no accessible earthed or uninsulated conductive parts?
Its function might require an internal earth but we are discussing safety testing.
Robert

The manufacturer states the device is Class 1 CF.

Concerning medical equipment, with applied parts, I think it does. What about applied parts leakage w.r.t earth?, for example, (irrepsective of whether there are accessible earthed parts on the case or not).

How do we know that applied parts on a Class 1 device with "Class 2 parts" are isolated from earth (or with mains on applied parts w.r.t earth) if we test Class 1 devices with "Class 2" parts as Class 2 overall?

Even if we test Class 2 and assume a "functional earth" there's still a requirement to test functional earth leakage and touch currents, etc, plus applied parts w.r.t earth on Class 2 devices is there not?

If we don't test Class 1 then this implies that there's no earth bond test performed on the 3-pin removeable IEC leads fitted to devices with "Class 2 parts", i.e. inaccessible earths.

In my opinion it's better practice to test Class 1 devices (with inaccessible but protective earths) as Class 1, mark the earth as inaccessible, then test the mains IEC earth bond seperately, followed by the tests actually required for a Class 1 device, rather than miss earth-referenced leakage tests, etc, because Class 2 EST is performed.

We must trust manufacturers and notified bodies to classify their products and then test them appropriately - precisely because we're not type-testing. Mains electrical devices are either Class 1 or Class 2 - nothing in-between as far as I'm aware.

A colleague in my department manufactured in line insulated boxes with and accessable earth point (standard 4mm socket).
We have been looking at the benefits of using this device and from a positive point of view we can check the earth continuity of the mains lead and can also monitor earth leakage current. Since there is no exposed conductive path then there seems no further benefit unless the device has a patient connection.

Any thoughts?

ps Orders can be placed via private message....

Thanks for all your contributions.

JohnBhoy - RL Dolby, who sold the uniits to us, have informed us that Criticaire are fitting earth studs to units in future manafacture, but not to units already sold.

The agreement is that we open the unit (and therefore break the seal stating warranty void if broken) and test, as a few of you have suggested to an internal earth point.


Mick