Thanks for that Ian,
My view regarding testing for earth-bond is that we should test for the worst-case or "poorest" earth on equipment by "probing" for earth points rather than choose one for repetitive testing. Anything that's accessible, conductive, and not earthed may be considered as enclosure (accessible conductive non-earthed parts - unless inspection indicates otherwise) and enclosure leakage/insulation resistance measurements can be taken from these points independently of the earth if need be.
Whilst I agree your method of marking earths for testing purposes can ensure that degradation in the earth bond can be observed using repeatable and comparable testing over time it doesn't necessarily ensure that the earth bond to other accessible earths hasn't degraded. There's always a problem with automated tests that use a single point for insulation resistance/earth-bond/enclosure leakage tests is not adequate, even for routine tests, because operators are not encouraged to "look" for potential problems.
By the way, we used to have battery chargers for Laerdal 911 defibs. that had no markings to indicate class I or II They had an earth connection at the 13A plug but a fully insulated enclosure (plastic) with no external metal parts, no external earth point or equal potential earth pin. The earth connection went to the inside of the charger and was totally inaccessable.
In my opinion this doesn't mean class 2 applies necessarily - markings must be provided to indicate that a mains device is class 2. If a device is internally powered at low voltage, e.g. a LSU, then it will take on the classification of the mains power source as far as I'm aware. If the power source had no class 2 markings, a conductive earth-pin, and no accessible earth on the housing then I'd always assume class 1 with inaccessible earth rather than an incorrectly marked class 2 PSU (that would fail to meet the requirements of MDD, incidentally, if it weren't marked appropriately).
Even if there's a non-conductive enclosure there's a requirement to give earth protection if the case/internal construction doesn't meet electrical and mechanical requirements for adequate seperation from mains (class 2 does hence no protective earth is required). The protective earthed parts that are physically inaccessible because of the enclosure are considered "accessible" if the case and internal seperation from mains parts do not meet requirements. There is nothing that says the protective earth has to be provided externally on class 1 devices as far as I interpret it.
Either way I'd try to confirm the classification before releasing the device into use. I think I posted some stuff in another thread
Acceptance Testing about the potential flaws in testing a class 1 device as class 2 but as you've already stated you've used this method for non-accessible earth devices only - but what if there were some accessible conductive enclosure or applied parts on the class 1 device that's tested as class 2?
The problem is that testing for class 2 on a class 1 device with inaccessible earth means that you still miss important safety tests that should be applied. So, personally speaking, I wouldn't recommend performing class 2 tests in lieu of class 1 under all but the most basic circumstances.
Earth leakage current is an important measurement in medical device EST that would tend not be measured under class 2 by safety testers following automated protocols, as far as I'm aware, Ian. Personally speaking, in the scenario you give, I'd be inclined to measure to class 1 and note "earth inaccessible" rather than test a class 1 device with a non-conductive case, to class 2 EST, and fail to establish if there's excessive earth leakage, for example.
Richard.
