Testing when modifying a circuit

[timhoward]

Of course I know it's done, but I'd almost consider thay a slightly greater risk than omitting an R1+R2 - especially if there's evidence of recent periodic testing.

It's particularly important to ensure that ADS (automatic disconnection of supply) will occur under fault conditions. The only way to be sure is to know that the CPC path has a sufficiently low impedance to allow enough current to flow to trip the protective device.

Worst case - no IR testing so you don't know you have a N-E fault in your new section, no R1+R2 testing so you don't know you have no earth path back, and you could end up with metalwork that's gives someone a shock if touched.

If I had an EICR showing a circuit was recently in fine health, and a board that was particularly problematic to work on for any of a number of reasons, at an absolute minimum I might do a live Zs before I start on the unmodified circuit, IR the new section, and do a live Zs when I finish. This at least tells me that there is earthing and no cable faults in my addition/alteration.
I'm not encouraging this sloppy practise but it's better than no testing at all.

 

[westward10]

Evidence of an EICR means little unless you have access to the Report. The cìrcuit you are working on may have Code 2 issues.

 

[lozarus]

It's particularly important to ensure that ADS (automatic disconnection of supply) will occur under fault conditions. The only way to be sure is to know that the CPC path has a sufficiently low impedance to allow enough current to flow to trip the protective device.

Worst case - no IR testing so you don't know you have a N-E fault in your new section, no R1+R2 testing so you don't know you have no earth path back, and you could end up with metalwork that's gives someone a shock if touched.

If I had an EICR showing a circuit was recently in fine health, and a board that was particularly problematic to work on for any of a number of reasons, at an absolute minimum I might do a live Zs before I start on the unmodified circuit, IR the new section, and do a live Zs when I finish. This at least tells me that there is earthing and no cable faults in my addition/alteration.
I'm not encouraging this sloppy practise but it's better than no testing at all.

Genuine question here - and I'm probably missing something obvious. You say no R2+R2 means no proof of earth back - surely a Zs measurement within the specification of the device proves there is?

Again I may be missing something here...

 

[lozarus]

Evidence of an EICR means little unless you have access to the Report. The cìrcuit you are working on may have Code 2 issues.

I commission the EICRs here - and know the results like the back of my hand. Appreciate that's only relevant in my scenario but still.

 

[timhoward]

Genuine question here - and I'm probably missing something obvious. You say no R2+R2 means no proof of earth back - surely a Zs measurement within the specification of the device proves there is?

A Zs certainly gives a strong indication all is well which is why I suggested doing this if a board truly can't be worked on. That said...

An R1+R2 is proving good connections and path along a specific CPC.
A live Zs test can prove there is a path back, but it could well be via metal trunking, bonding and a copper water pipe.
But as above, it gives some reassurance and is a lot better than nothing.

 

[lozarus]

A Zs certainly gives a strong indication all is well which is why I suggested doing this if a board truly can't be worked on. That said...

An R1+R2 is proving good connections and path along a specific CPC.
A live Zs test can prove there is a path back, but it could well be via metal trunking, bonding and a copper water pipe.
But as above, it gives some reassurance and is a lot better than nothing.

Thanks. This really has been useful. Can definitely see the pitfalls to relying on live Zs. I think I'll get cracking on writing up some procedures.

In the meantime there's a few jobs coming up on site - mainly adding some FCUs for door controllers. All fed from small MEM2 boards. Think I'll undertake full testing of these to give me some support in my studies. Thanks for the help as ever guys.