Parallel RCD Testing

[CPL]

I've got to test a bunch of temporary electrical distribution boxes (see photo example) for an events company I work with. I've done this many times before and have no issue with the actual testing. However, normally I would do it on the bench with a non-RCD'd supply so as I can test and record RCD trip times for each RCD or RCBO individually. Unfortunately I'm having to perform the testing on site, where ALL supplies are protected by 30mA RCD's, which means there is no discrimination between devices and when testing, can trip both or just the upstream RCD rather than the one I'm testing.
I've heard about using a 230v/230v transformer to separate the primary and secondary side RCD's. However I'm unsure of the type of transformer required for this...
Surely a 230v separating transformer, having no secondary side connection to earth, would not allow an RCD to trip?

Does anyone have any ideas or links to solutions that would work in this situation?

There will be no load whilst testing so I was just going to use a 13A plug top, jumped up to 3ph ceeform with the phases linked, I just need a way of separating the RCD's...
TIA

 

[westward10]

A 230/230 isolating transformer should do the trick as it isolates the rcd you are testing from the source supply, not cheap though but you could possibly hire one.

Carroll & Meynell Isolation Transformer, 230/230V, 3kVA, 2 x 13A UK sockets - CM3000/230

Carroll & Meynell Isolation Transformer, 230/230V, 3kVA, 2 x 13A UK sockets - CM3000/230, , , Description, , , Manufacture to BSEN 61558, Thermal ove..

www.sitebox.ltd.uk

 

[James]

I am a little confused about your comment about linked phases in plug.
Can you elaborate a bit?

 

[Strima]

I am a little confused about your comment about linked phases in plug.
Can you elaborate a bit?

He means linking all the phases together in a three phase plug to run off one phase.

 

[James]

Well I hope they are linked in a 5 pin socket attached to a single phase plug.
Other methods could be a bit too flashy and bangy if plugged into 3 ph supply

 

[Lucien Nunes]

Surely a 230v separating transformer, having no secondary side connection to earth, would not allow an RCD to trip?

That's true when it is being used in its usual application to provide an earth-free IT supply from the secondary. However, if you link N & E on the secondary side, you will have a regular TN supply for testing the RCDs in your distros. The upstream supply RCDs will not trip because the test leakage is returning to the secondary of the transformer, not the actual site supply, and does not create an imbalance in the primary circuit.

I prefer not to make the N-E link inside a regular blue-cased transformer in case anyone misses the warning sign and expects the output to be earth-free, as this could lead to serious shock hazards. I have a jumper cable with the link, which still has to be clearly marked but would not lead to the same level of risk if misused. Alternatively, since the isolation is only for functional purposes, not safety, and the load to be powered is minmal, it's not necessary to use a blue transformer; anything with 230V of secondary will do the job. I just happen to have a few about.

Other methods could be a bit too flashy and bangy if plugged into 3 ph supply

It's a pretty standard bit of kit in events wiring, for powering things up via 3-ph distribution when only 1-ph is available. Most loads are actually 1-ph, so they all work subject to only 1/3 of the total current being available. Anything truly 3-ph such as motors simply doesn't work.

 

[Wilko]

Hi - an RCD can be tested in isolation from the upstream RCD by creating a temporary “TNCS” test environment local to the RCD to be tested. The tester E lead goes onto the upstream N so any test leakage current rejoins N and is not seen by the upstream RCD.

 

[Lucien Nunes]

This is a good point and it's how I would do things from an open DB. However, I was envisaging the OP working on a ready-rigged temporary installation where the covers are not being taken off and the upstream N is available only by adding a breakout point into the circuit. On further thought, it's about the same amount of effort to do this as to slot a transformer in, you just need a plug-to-socket jumper cable with the N brought out to a 4mm test lead socket.

 

[CPL]

Thanks for all the comments guys. Much appreciated!

 

[R-fur]

I think wilco has the best solution for you, but if you go down the transformer route it may be cheaper to reverse a 110v site transformer (only if you have 110v site supplies obviously) I use one to provide 230V test supplies when only 110V is available.
Also I don't see the point of linking the phases in a three phase plug. Apart from the dangers that Lucien points out what are you gaining? The RCD has a common core and detection circuit so AFAIK it is normal practice to test three phase RCDs on any one phase only.