The isolation transformer adds the second isolation to make it double insulated.
Either way there always need to be two means of protection against
accidental contact, for example basic insulation and earted chassis,
or double insulation (and no earth).
Of course you can always have more means of protection for extra safety.
ST
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On Sat, Nov 24, 2018 at 11:24 AM Jack <goldmort@...> wrote:
Removing an earth does not make anything doubly insulated unless it already was doubly earthed . Earthing the case of an isolating transformer is not the same as connecting it to the chassis of the appliance, which is now being effectively fed by an entirely different supply from the submains which supply the transformer. That's its design criteria and intent.
When the case is earthed (some have a transformer screen which is earthed) to maintain safety it should be in a situation where the person (whilst) working on the set cannot contact it. We don't wand p.d between him and any earthing.
Standards are the minimum requirements and where introducing hazard, can be overcome (to some degree same as with codes of practice) by a properly and competent qualified person so long as no other hazard is introduced. Sensibly any mods should be clearly marked. Unsafe light fittings are numerously introduced to the "West" and I have had to rebuild them..if not I wrote a defect sheet to the client and to the lights. I don't need permission to do my job properly. That includes ensuring correct polarity and switching.
If an isolation transformer has earth and neutral connected I 'lift' it as it's 'not on' to have that situation as an "MEN' and I earth the transformer case with no connection to the neutral....but the case must be kept separate from appliance. Earthing the case is to enable protective devices to act where there might be transformer breakdown to case, or some wire coming free and contacting the case. Of course one can introduce an RCD arrangement if up on that, once the earth/neutral linkage is removed . Best practice safety is the objective. As for permanent isolation connection....I covered that and now 'I'm out of here'. Good luck....Voila. ..
--
Jack
