Testing on system changes

[SimonH2]

Not long ago I witnessed a registered sparky upgrading a circuit - the RCBO for the garage circuit was tripping when starting tools. Existing circuit, recently part of a partial rewire with new all-RCBO CU, garage supply split off from RFC.
So having swapped out the RCBO for a different rating and trip curve - what testing would you expect to do/see done ?

 

[EFLImpudence]

the RCBO for the garage circuit was tripping when starting tools. Existing circuit, recently part of a partial rewire with new all-RCBO CU, garage supply split off from RFC.

Do you mean it is the RFC (I presume you mean Ring FC as R stands for Radial as well) RCBO that is tripping?

If so -

So having swapped out the RCBO for a different rating and trip curve

What rating was the RCBO before and after?

If not, what do you mean?

what testing would you expect to do/see done ?

Ensure bonding is satisfactory,
ensure the Ring is satisfactory, continuity of conductors, Zs, etc.
new RCBO operation and trip times.

 

[ban-all-sheds]

I would expect loop resistance to have been checked before changing to a different type of device, cable current carrying capacity to be checked before changing to a different rating, polarity, RCD tripping afterwards...

OTOH, given your post, I would expect to find that they only did the SFA test.

[confused]

RCBO for the garage circuit... all-RCBO CU, garage supply split off from RFC.

What does "split off" mean?

If the garage circuit is spurred from a ring, that would not go via an RCBO, nor could the RCBO for the ring be changed to a different rating. Nor probably a different type, unless it was a low Ze TN-C-S supply.​

[/confused]

 

[ericmark]

The cable size, and loop impedance could be all given in the installation certificate but he would need to test the RCD part and complete the minor works certificate.

 

[John D v2.0]

It's effectively a new circuit using all the existing cabling so for the changechange checking the cable size and installation method, also the R1+R2 and ze to calculate whether the RCBO will trip magnetically in the event of a fault at the extremity of the circuit. Then as said above the RCD part would need testing after installation.
I'd also expect the polarity, IR, and Rn to be checked as it's a new circuit in effect, although they aren't really relevant to the change of rating.
Eric is optimistic relying on the installation certificate if isita domestic property!

 

[SimonH2]

Sorry I wasn't clear. Originally the garage was just tee'd off the ring final (JB under the floor), during the works this was split off and made it's own radial with a separate RCBO. I assume all the relevant tests were done as part of the partial rewire and new CU, but I've not seen any paperwork (I wasn't really involved in the project).
Latest change was to swap the B16 for a C20.
Yes, I'd have expected his to re-check at least the fault impedance and test the RCD (didn't even press the test button !). Sole test was to plug in his handheld circular saw and check that it would start without tripping
When I get chance I plan to go and make some measurements, and calculate if a C curve is suitable.

 

[JohnW2]

Latest change was to swap the B16 for a C20. ... Yes, I'd have expected his to re-check at least the fault impedance and test the RCD (didn't even press the test button !). Sole test was to plug in his handheld circular saw and check that it would start without tripping

As BAS has said, he surely have measured the loop impedance before changing the RCBO, to make sure that it would be possible to change to a C20?

Kind Regards, John

 

[ban-all-sheds]

Sole test was to plug in his handheld circular saw and check that it would start without tripping

given your post, I would expect to find that they only did the SFA test.

 

[SimonH2]

One would think, or at least hope, so - I assume he'll have referred to his test results from when he altered the circuit and put the new CU in. But I haven't seen the test results.

 

[ericmark]

Sorry I wasn't clear. Originally the garage was just tee'd off the ring final (JB under the floor), during the works this was split off and made it's own radial with a separate RCBO.

Then it is a new circuit, no if's or but's it is a new circuit, as such it will have a new installation certificate with all tests done, and it will have either a completion or compliance certificate in England and Wales.

I find filling in the installation certificate is a good reminder of what needs testing.

 

[SimonH2]

Yes, that was done earlier (partial rewire of the property and a new CU) - I assume the paperwork and testing was done etc. I wasn't there so didn't see.
This was about what testing would/should be done when replacing the RCBO for the circuit. I'd have expected to see him at least test the RCD - he could have referred to his earlier test results to determine if a C curve was suitable.

I find filling in the installation certificate is a good reminder of what needs testing.

Yes, I imagine it is. Nothing like a good checklist to avoid missing things.

 

[JohnW2]

I'd have expected to see him at least test the RCD - he could have referred to his earlier test results to determine if a C curve was suitable.

To delay the Zs test until after changing the RCBO (on the basis of previous tests) requires considerable faith that (a) nothing has changed since the circuit was installed and (b) the documented measurements when the circuits was installed were correct, if one is to avoid the risk of having to change back to the Type B RCBO!

Kind Regards, John

 

[John D v2.0]

Not sure how considerable faith has to be, it doesn't take long to change an rcbo once you have the wires oro for testing, so it's a toss up between time taken to get the tools out of the van or just swing by the wholesaler on the way past.
Admittedly rcbos are a lot more costly than MCBs, but I'm guessing most electricians have a small stock of common makes.

 

[JohnW2]

Not sure how considerable faith has to be, it doesn't take long to change an rcbo once you have the wires oro for testing, so it's a toss up between time taken to get the tools out of the van or just swing by the wholesaler on the way past.

Yes, but given that one is (hopefully - even if that didn't happen with Simon's!) going to have to get out one's tester to measure Zs of the circuit after the change, it would surely only take a few seconds to make a measurement before the change and that means that, if the Zs proves to be too high for a Type C, one wouldn't even have to spend time opening up the CU?

Kind Regards, John

 

[SimonH2]

To delay the Zs test until after changing the RCBO (on the basis of previous tests) requires considerable faith that (a) nothing has changed since the circuit was installed and (b) the documented measurements when the circuits was installed were correct ...

I don't think that's too much of a problem. He was the one that did the CU and partial rewire, and it was only a week or two since it was done - changing this breaker being one of those "snag list" items to complete the job. I asked the person who dealt with the sparky today - the certificate/test results haven't come through yet.