Discussion in 'Electricians' Talk' started by tina lucinda lane, Jan 13, 2021.
Yeah that had also been my thought
It usually passes through but the output is dereferenced from the neutral-earth link on the primary side. By joining two transformers back-back as @MGW did, you are essentially creating an isolation transformer.
Here is a good website that explains:
Just spend to £60 and get the kit to do it correctly. Cheaper than a new circuit.
I can get the new circuit installed for £15 as well it at shortest will only run about a metre before becoming a socket, rest then legally can be installed by myself as its temporary in the interest of the law as its supplied by a plug if I'm not mistaken
As, the kit doesn't guarantee that it won't trip a main rcd
So you’re then back installing a socket supplied from the transformer with the earth connected to the neutral, so is there an advantage in having the the transformer?
Isolation transformers for tools are earth free, but referenced to earth, so you need to introduce an earth to test the RCD.
Look at the diagram in the link you referenced to.
There’s the diagram from the link that was posted, the second transformer being the isolation transformer.
So just how would you connect a RCD to the isolation transformer to test it?
Presumably by making a neutral to earth link down stream of the isolation transformer to connect a socket to plug the RCD into?
That's what it's designed for.
Isolation transformers like that pass the earth from input plug to output socket. They were commonplace in repair workshops for TVs etc. No need for an additional socket, just plug the tester into the transformer secondary socket.
These diagrams are more relevant from the webpage I linked to, in particular the second:
The imbalance to test the plug in RCD is introduced by the tester and doesn't have any relevance on the deferenced neutral-earth link.
£15? Umm. Doubtful. I can't see a spark wanting to allow it either.
It does, they're used all the time. That, or construct your own test lead as described. Most sparks have that in their van.
He's family so family rate
But this has all been useful as it gives me great options that I hadn't considered thanks everyone
Just to be clear, we are talking about RCD testing as part of PAT testing, in particular these Plug In RCDs
The test button is wired back to the input terminals through a resistor, like it is in a fixed installation RCD, so a test button can test the RCD without a reference to earth to create an imbalance.
When you plug a RCD tester in to one of these it doesn’t have access to the input terminals, so has to use the earth to create the imbalance and that trips both the plug in RCD being tested and the installation RCD that is upfront of it.
To get the RCD tester to work downstream of the isolation transformer you would have to wire the test socket with an earth connection to the neutral as in figure 3 above, the one where is says the operator is unsafe
But you don’t actually then need the connection to the real earth from the downstream side of the isolation transformer.
However once you have made that connection to neutral you don’t actually need the isolation transformer as the upfront RCD won’t trip anyway whilst you are testing and if you don’t fit the isolation transformer you are actually still protected by the installation RCD.
Just to add to that, it could be a lot easier depending on how the test lead is connected to the RCD tester.
If you have test leads like this
buy an additional set, then use one set to plug the live and neutral from the RCD tester into the Plug In RCD you are testing that is plugged into one side of a double socket, then use the second set to plug the earth from the RCD tester into the neutral of the other side of the double socket.
I should have told you that yesterday
I was also taught to always do an earth loop test before doing a RCD test, because of the current you are introducing into the earthing system, which may be hazardous to people using the installation.
You are using lethal currents to test RCDs that runs out through the earthing system, which is why when I’m testing other people’s equipment at home I’d rather run the test current out through the neutral, whilst maintaining the RCD protection at the socket.
Yes that type and the plug version as well as extension leads with and in built rcd at there socket end (rare I know) as well as one's fitted with an inline rcd on them as well btw if it helps this is the rcd tester I own robin digital rcd tester kmp5404dl which has both an iec socket on it also has a lead to provide a reference to earth as well hence the need for a method to use it without tripping a main board rcd and knocking out part or all of an installation
Using a shaver output you have a IT supply so no problem tripping, the question is if the 200 VA is enough for your tests.
But the question is if only using line and neutral you know you can make a rig to test the RCD so you just keep the rig in your tool kit, same as I have an old BA22d to 5 amp adaptor, very handy for testing lights, and it never leaves my test kit.
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