My own preferred option is to have a fully isolated and floating supply for feeding the equipment under test, and if necessary I would plug my scope or other piece of test equipment into the same isolated supply.
When using multiple items from a single isolated
supply any CPCs (protective earths for the test equipment) should be connected together but not earthed
, the easiest way of achieving this is to have say a 4-way socket block connected to the secondary side of the isolation transformer (no earth) and plug both the item and the test equipment into the same socket block.
Now the above is not ideal, as it is possible if using two items of class 1 test equipment to create a short via the CPCs supplying the test equipment and any earthed leads, or cause other problems say by monitoring both the hot and cold sides of a SMPSU, I would mitigate the risks here by using only one piece of test equipment at a time, and being careful where you connect the test equipment to.
I found when using my Hameg scope on this type of set-up to just use one earthed clip off of the scope to attach to the equipment under test, even if requiring both channels.
This set up is a reasonable compromise and is about as safe as you can really expect using a simple set-up.
As Jeffrey pointed out above, by using a DIY TNC-S type of ISO TX and RCD combination you lose the safety measure of electrical separation and end up with RCD protection in lieu, to me by using that method you are back to square one really, and may as well just use an RCD protected outlet directly off of the mains.
About the only thing this may achieve is a lower prospective fault current via the ISO TX.
You are merely replacing one protective measure with a different one (you cannot have both).
The exception being of course is if you were using this type of supply for testing RCDs.
Jeffrey while you are correct about the DIY type supplies,
by ppppenguin » Fri Apr 11, 2014 7:11 am
We can use an isolating TX to provide homemade versions of various supplies.....Completely floating is IT. Earthing one side of the secondary is TNC-S or TNC which amount to the same thing here
I found that this was common, even industry standard, in say control panel transformers and the like, but this was limited to being part of the equipment itself rather than actually being distributed for use elsewhere. The main reason the secondary's were earthed in this case was for fault protection to the cables rather than for shock protection.
I would really need to think more about that one
by Alistair D Currently I am tryng to read up about the properties of the TNS/TNCS earthing systems to find out why they allow an RCD to trip but my locally made earth neutral join wont. I understand why the local join does not allow the RCD to trip but not why the household earthing arrangement does.
Al, I mentioned this earlier but I will repeat it again in case you missed it, the RCD requires some current flowing through it L and N for the trip to work, your house RCD probably has some loading already present, Try say running a lamp from the out going side of the RCD while trying to trip it.
Also on this set-up the CNE link is very close physically to the RCD.