I have removed the lid from my 1kva yellow site tansformer, and have noticed two unusual things: The earth pin on the 110 volt socket is not connected, is this a mistake by the manufacturer? I have also noticed the transformer casing is filled with a fine sand type material, with a layer of resin to hold it in, this has shunk away from the edge leaving a small gap. The sand starts to come out through the gap when the transformer is tilted, is this the normal filling for these transfomers?
It should be connected. To the earth terminal and a connection at the cable outlet
The filling of "Sand" is normal. It sounds like the transformer has been shaken or dropped causing the inside to separate from the case. If you are worried about it rattling just pour glassfibre resin into the gap around the edge to secure it.
NO! This is site isolating transformer it is producing a balanced and floating supply. The theory is that either phase can be touched (but not both!) and not receive a fatal shock as the touched phase just becomes your potential as there is no reference to any earth/ground.
Are you sure about that - can you quote any standards, etc.? I was under the impression that these transformers usually provide 110 V with an earthed centre-tap (55-0-55 V). Certainly if you're providing such a supply on a building site as part of a semi-permanent installation you would have to earth the centre-tap - see Reg. 604-02-02 (iii) in BS 7671.
I knew someone would ask that, I had a quick google but couldn't find anything definative.
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a vauge overview of the reasoning between true isolation and earthed center tap transformers.
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an "consumer" overview but with references to the relevant regs. I find it unclear between the text where, for use with single appliances, it says no connection to any other circuit or earth but the diagram shows an center tapped earth and a connection to protective conductor (no indication as to where that protectice conductor is connected though. Perhaps the diagram is for supplies to more than one bit of kit or through hidden wiring. I can see that the center tap and earth is probably safer in this case (hidden or remote short to earth on one phase, causes other phase to be 110v wrt to earth). For a single appliance connected via a visible flexable cord the fully floating and isolated supply is probably safer.
That is NOT the same as connecting the earth pin of the 110v outlet to the 240v supply earth or the transformer casing.
Again it's the transformer center tap that is earthed not the outlet(s) earth pins. Agreed you now have 55v wrt to earth on both phases, rather than a true floating supply.
And (FWIW) says that tool transformers are usually of the CT & earthed variety.
Again this is the BS 7671 perspective for fixed wiring. What we really need is a product standard for portable tool transformers.
So, following the BS 7671 approach, it would be OK for a tool transformer to have a separated (floating) o/p if, and only if, it had a single o/p socket. In this case the earth pin of the o/p socket should not be connected [per 413-06-04]. OTOH a transformer with multiple o/p's should have an earthed CT, also connected to the earth pins of the o/p sockets.
Probably, the whole issue of earthing is a complete mine field.
The OP said "The earth pin on the 110 volt socket is not connected, ..." which implies a single output hence it is correct not to have the earth pin connected. Provided that the connection between the appliance and the transformer is not hidden (or fixed?).
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