Earth Leakage Issue?

[Snapester]

Changed a CU about a month ago to a dual RCD but it keeps having intermittent tripping on the 2nd RCD, before changing it i did a full test and all IR was very good @ >999Mohm range no faults found.

I tested the leakage with my clamp meter and recorded a value of 34mA at the incoming tails, on the RCD in question i recorded 30+mA.
One circuit was the Solar PV panel and the other a Ring Final supplying a far building office which has about 6 PC's, UPS's & Printers and Servers etc!
After taking away the office sockets or PV Panel the tripping stopped?
So as the Client is worried about data loss and computer failure due to power loss i installed a second CU to feed the main office.

The question is should i put this office back on to RCBO in the second board or can it be left as a deviation from BS7671?

Regards
Ian

 

[malcolmsanford]

How are you supplying the far building. If the sub main is not buried and does not contravene regs 522.6.5 to 522.6.8 then you have no need to fit it with an RCBO.

The worse case scenario is that if your earthing system is TT, then the submain may need to have 100mA protection for the distribution circuit.

You could also look at running the PV installation surface which would negate the use of RCD protection unless the invertor can not feed DC fault currents into the AC side, or provide seperation, these are outlined in Regs 712.411

IMO unless your MCS I would not touch the PV side of things, but concentrate on the office. Can you split the load within the CU, ie the PV on one RCD the office on another. That would split the high protective current appliances.

IMO if the installation requires RCD 30mA protection either through the cable installation or socket outlet then no I would not omit that protection and mark it as a deviation, as if anything happened you would need to explain the thoughts behind the omission.

I would look at trying to take that submain off the RCD inside the house, onto either an unprotected way in the CU or even fitting a switch fuse via Henly blocks just for the office

 

[Snapester]

Basically the board is pretty balanced as there are some other high leakage loads on the other side also like the kitchen and living rooms areas.
There is no submain to the offices basically the offices has its own ring final which is the circuit in question, there is no chance of running outdoor portable equipment off these sockets but i cannot say whether that cables are protected in the walls so i'm guessing it should really have an RCBO protecting them.
I have just answered myself haha i guess i should just do what i know is right?
Are there any points in an installation that wouldn't require RCD these days!!

 

[malcolmsanford]

Very few in domestic. The office sockets can not even come under 411.3.3 for a single labelled socket outlet as it can only be one socket.

The best bet really is to somehow fit the circuit supplying the office on it's own dedicated RCBO which is seperate of the main RCD, which may need you to look at fitting a small sub board that is supplied seperate from the main CU or an unprotected side of the CU if there is one.

 

[nickblake]

its a difficult one this one what you could do is change the sockets to non standard how ever doing this still doesnt cover the cables less than 50mm in the walls , think the real way is to do as malcom has said by putting the circuit on its own RCBO , or install additional circuits then the earth leakage will be lower , this is what id do install some dado trunking with non standard sockets into 2 circuits surface mount the cables then No RCD will be required

 

[Chr!s]

411.3.3

Exception is permitted for socket out lets under the supervision of skilled or instructed persons

522.6.6 and 522.6.8

Exception is permitted where installation under the supervision of skilled or instructed persons.

Also Reg 543.7.1.3 High protective conductor currents >10mA

 

[malcolmsanford]

411.3.3

Exception is permitted for socket out lets under the supervision of skilled or instructed persons

522.6.6 and 522.6.8

Exception is permitted where installation under the supervision of skilled or instructed persons.

Also Reg 543.7.1.3 High protective conductor currents >10mA

I agree Chr!s if this was commercial or industrial, but I took it that as it was a CU change it was an office within a house ie someone working from home, not sure if there are children in the house that have access to the office if you could use that.

 

[Chr!s]

I agree Chr!s if this was commercial or industrial, but I took it that as it was a CU change it was an office within a house ie someone working from home, not sure if there are children in the house that have access to the office if you could use that.

Also the equipment will need to be connected in accordance with 543.7.1.2, this in its self will lessen the risk.

 

[malcolmsanford]

Agree mate but would it overide the use of RCD protection under 411.3.3 and use by non skilled or supervised persons, it's certainly a good debate lol.

Again though the office might not be producing more than 10mA, he did say that it was a combination of the office and the PV installation and also other equipment.

I think it will need to be split into seperate circuits as in 314.1 (iv)

 

[Chr!s]

Agree mate but would it overide the use of RCD protection under 411.3.3 and use by non skilled or supervised persons, it's certainly a good debate lol.

Again though the office might not be producing more than 10mA, he did say that it was a combination of the office and the PV installation and also other equipment.

I think it will need to be split into seperate circuits as in 314.1 (iv)

Agreed, a little more investigation required.

Also 531.2.4

 

[Martin Bradley]

Pc and some other it equipment have a natural earth leakege up to about 3.5 mA per pc, this could be the problem?
you may have to opt for rcd sockets.
remeber some rcd breakers trip at 21 mA even if it does say 30mA on device.
perform an RCD test to see.

 

[Engineer54]

Almost all computer equipment has high leakage currents, compared with normal appliances. In this particular case, where the office installation is within the confines of a domestic installation, the best, or should i say the easiest solution would be to install RCD outlets within the office area as suggested above. Could be an expensive solution!!! ...Although, i'm not sure if according to the 17th ed this would negate this daft ruling of cables needing to be buried >50mm or not?? It will still need a non RCD protected supply, hence a separated sub-CU...

Yet another example of the Reg's tripping it's self up, when it comes to RCD requirements...