RCDs raising zs values

[dawsirn]

Was at a property toady which I've done previous work before including fuseboard change. All i did today was change a few socket plates over for new ones. Took some Zs readings and noticed they were very near the limit of being acceptable ie: 0.92, 0.98 etc.
Took a Ze at the board with main earth disconnected 0.06
reconnected and took various 3 wire no trip loop tests at different positions in DB. basically i was getting nice low readings on the supply sides of rcds but on the load side of both of them i was getting about 50ohms higher.
The RCDS are MK and only fitted about 6 months ago and when originally fitted tested out fine.
I also ran through standard RCD tests to see if they were ok which they were. ZS results were still high after this too.
Also checked busbar and neutral leads were properly connected in the load side of rcds, which they were.

Never have this happened before, didnt know if anyone else had. Do you think RCD replacement could be required?
cheers

 

[DJ Ovengloves]

Why do you think those values are near the limit go being acceptable?

 

[Sintra]

That's why I say a live Zs test is a must on all circuits as I have seen lots of devices with high internal resistances.

 

[dawsirn]

Why do you think those values are near the limit go being acceptable?

32a type b mcb max 1.15ohms(80% bs7671) . i didnt test every socket on that ring and i know theres a few spurs which are most likely going to be higher than 1.15ohms.

Even if they didnt exceed the limit i'm still curious to why the RCD raised the zs values so much higher.

 

[DJ Ovengloves]

I get that, what I meant was what was:-

What was the reading before you swapped the faceplates?

How long is the cable run and therefore the expected resistance?

Are you taking the reading through the socket or at the terminals?

Etc

Why do you think those values are near the limit go being acceptable?

32a type b mcb max 1.15ohms(80% bs7671) . i didnt test every socket on that ring and i know theres a few spurs which are most likely going to be higher than 1.15ohms.

Even if they didnt exceed the limit i'm still curious to why the RCD raised the zs values so much higher.

 

[Engineer54]

I don't think he has a problem with the circuits wiring Zs, he is experiencing a common occurrence of protection devices developing a high internal resistance.

 

[spark 68]

I don't think he has a problem with the circuits wiring Zs, he is experiencing a common occurrence of protection devices developing a high internal resistance.

Not all of these protection devices have a 'real' high impedance though, part of the problem is the No-trip setting and the testing method used by that particular instrument, which depends on the particular instrument used and to a certain extent the device itself.

I had one once that added 7 ohms measured, this was not possible however as I double checked by measuring the voltage drop across both poles individually while pulling a heavy load, and no way was the device introducing a 'real' 7 ohm drop.

 

[DJ Ovengloves]

Agreed, but best to check

I don't think he has a problem with the circuits wiring Zs, he is experiencing a common occurrence of protection devices developing a high internal resistance.

 

[dawsirn]

I get that, what I meant was what was:-

What was the reading before you swapped the faceplates?

How long is the cable run and therefore the expected resistance?

Are you taking the reading through the socket or at the terminals?

Etc

yeah sorry should have made it clearer.
I tested two separate rings protected by different rcds. Used the socket lead to test sockets on the no trip setting. readings at old sockets and throughout the house were fine when i initially did the fuseboard change a few months back.

but this is all a bit irrelevant because it was the zs readings either side of the RCD inside the consumer unit is where i found the 50ohm difference in readings. I don't know if it is just the fact that the no trip setting caused the high readings but i've never had this issue before especially on both RCDs! If i get a chance i may swap one of the RCD's just to see if that makes a difference.
cheers

 

[baggy61]

Fitted some cheap W.F own brand sockets the other week,r1, r2, rn were fine as wereR1+R2,, Zs on lights ok, Ze ok,Zs on terms ok but about 0.20 ohms higher on plug in tester. (yes I had zeroed the leads.
Regards.

 

[spark 68]

I would think this is a phantom reading (see my earlier post #7), although I have not had one that high (my highest was 7 ohm), a 50 ohm resistance across the RCD would give you a 50V drop across the RCD when pulling 1A!

Have you tried measuring the Zs at a socket with the plug in lead again ?

I think it was a MK RCD that gave me the 7 ohm reading.

 

[Lucien Nunes]

50 ohms? Do you mean 0.5 ohms? Might be worth checking the RCD dead on low ohms resistance range to see how much of that is contact resistance and how much due to the way it affects the loop impedance test accuracy.

 

[FAA Greenie]

I've had the same issues with MK RCDs luckily they all stayed within limits.

Rubbish I don' use them anymore.

 

[dawsirn]

50 ohms? Do you mean 0.5 ohms? Might be worth checking the RCD dead on low ohms resistance range to see how much of that is contact resistance and how much due to the way it affects the loop impedance test accuracy.

oops, sorry i do mean 0.5 ohms!