Another RCD problem

[jibjob]

Fitted a standalone fused 30mA RCD unit today protecting a spur from existing circuit wired directly from MCB in CU. RCD FCU is mounted adjacent to CU. CU has 100mA RCD upfront (Crabtree) & is populated by 60898 MCBs. 30mA RCD x1 test was fine (28ms) but the x5 test trips 100mA as well as 30mA RCD with a result of 41.80ms. Test is carried out from new socket outlet. Furthermore, if the 100mA RCD trips, upon resetting it the 30mA RCD trips of it's own accord.
Anyone have any ideas as to what might be occuring? Cheers

 

[Spazz]

Is this a domestic property?
if it is why have you got a 100mA RCD - change this to a 30mA and forget the 2nd RCD - the 1st will protect it.

Easier and cheaper for you and the customer.

 

[polo1]

Is this TT? I'm guessing the 100mA RCD is not an S type?
Regards

 

[jibjob]

Is this a domestic property?
if it is why have you got a 100mA RCD - change this to a 30mA and forget the 2nd RCD - the 1st will protect it.

Easier and cheaper for you and the customer.

It is a domestic property, 100mA RCD already in situ, one of the older style RCDs (big chunky thing that takes up about a third of the board). Felt it would easier to fit standalone RCD protecting part of install I'm concerned with rather than trying to source an old 30mA Crabtree chunky which would ultimately effect entire install.

 

[jibjob]

Is this TT? I'm guessing the 100mA RCD is not an S type?
Regards

TNS, not S Type

 

[Spazz]

What dont you change the whole board?
You can get fully loaded ones for less than £70 now - MK as well if that's the quality you want (Yes I have seen the posts about MK boards)

I have found that fully loaded CUs are now cheaper than some RCDs.

 

[polo1]

You won't get discrimination between the two RCDs on x5.

 

[truckster]

X5 on a 30mA is 150mA so it will trip a 100mA

 

[jibjob]

What dont you change the whole board?
You can get fully loaded ones for less than £70 now - MK as well if that's the quality you want (Yes I have seen the posts about MK boards)

I have found that fully loaded CUs are now cheaper than some RCDs.

If it was my own property then I probably would but it's not. At the end of the day the customer wanted an additional socket in their hallway which I provided within the regs, all tested etc and at about a quarter of the cost to fit & test a new CU.

 

[jibjob]

X5 on a 30mA is 150mA so it will trip a 100mA

Ok, I guess if the 30mA RCD is tripping at 28ms at x1 then it's operating as intended. Any idea why the 100mA trips the 30mA when reset?

 

[Spazz]

Remove the 100mA RCD and test it that way - then re-install the 100mA RCD.

You cant guess at that, the EIC requires exact readings, if you guess at it, its not worth the paper its written on, and your neck is on the line if the RCD is faulty

 

[truckster]

Presumably there is a slight difference in the phase/neutral closing contact of the 100mA rcd sufficient to operate the smaller rcd which sees it as a lost neutral

 

[Mr.Ian]

Presumably there is a slight difference in the phase/neutral closing contact of the 100mA rcd sufficient to operate the smaller rcd which sees it as a lost neutral

Do RCDs trip on an incoming neutral loss? I thought they only checked their load side.

 

[Spazz]

they do only trip on the load side, but you can have extra current flow in the neutral return and that will trip it.
The RCD is looking for a difference of (100mA for a 100mA RCD or 30mA for a 30mA RCD), the difference can be on the line or the neutral, but if there is a difference then it will disconnect the supply.

 

[telectrix]

could just be a slight few millisecs between the 2 poles of the 100mA making contact.

 

[Mr.Ian]

I'll try it out just to see out of curiosity, but I would've thought unless there was a fault on the load side of the RCD then it wouldn't trip. If it's not supplying anything (no load) the RCD still has 230v on one side and 0v on the other yet it doesn't trip, when there's an inbalance under load it will trip. I still can't see really how the supply could trip it.
I am a bit tired mind you and could be having a senior moment !!!!!!!!!

 

[Spazz]

Im thinking as the jibjob said the supply was knocking the RCD out, but when the MCB is turned off, cutting the power (also the isolator is off) - there must be a earth leakage between the CU and the isolator on the line side.

If this was on the neutral side the RCD will still be tripping out.

The other problem might be an issue with the cpc on the cooker circuit - do a Zs on that as well and a R1 + R2.

What supply do you have? TNC-S??