RCBO Fault

[TripleD]

Hi,

Anyone have experience in the following:

16a MK RCBO Tripped supplying Garage. 2 Circuits. Radial and Lights (6/16) MCB's

PME supply. Garage supplied in 6mm Twin and Earth with the Earth connected at the Main DB and Connector Blocked in the garage (doing nothing). Earth rod to garage in 6mm Multistrand and connected.

Twin and Earth IR Line/Neu +999, Neu/Earth +999 and Line/Earth 0.01. Removed Earth from main Cu and RCBO stable and live.

Zs 46.6 using just the Earth Rod, connected at the sub board in the garage. RCBO test sequence all clear too. Plugged in Freezer and RCBO tripped. Re-set RCBO and the Freezer is now stable.

2 Questions

Do you connect the CPC on the T/E at the MAIN Db when using RCBO's to supply a garage PME supply?
Any ideas why the freezer tripped the RCBO initially when plugged in for the first time? Leakage currents? RCBO ramp test was 21.0mA

Cheers

 

[telectrix]

might have tripped on the MCB part due to inrush current. might be an idea to replace with a type C.

 

[buzzlightyear]

PME supply. Garage supplied in 6mm Twin and Earth with the Earth connected at the Main DB and Connector Blocked in the garage (doing nothing). Earth rod to garage in 6mm Multistrand and connected.

The begging question why is the earth connected to the earth at the main board
And its a TT in the garage Is their any water pipes in the garage?

 

[Strima]

The CPC for the distribution circuit should be connected to earth at source.

How old is this freezer? Could be its just getting old and drawing too much current on start up. A standard domestic freezer should work quite happily on a B curve MCB.

 

[snowhead]

Twin and Earth IR Line/Neu +999, Neu/Earth +999 and Line/Earth 0.01. Removed Earth from main Cu and RCBO stable and live.

Is that statement correct?

If the freezer hadn't been plugged in for a while it may have a fault on it when the compressor is cold and now powered it's gone.
However it may return if it's ever turned off or it gets cold in the garage.

 

[Dartlec]

Hi,

Anyone have experience in the following:

16a MK RCBO Tripped supplying Garage. 2 Circuits. Radial and Lights (6/16) MCB's

PME supply. Garage supplied in 6mm Twin and Earth with the Earth connected at the Main DB and Connector Blocked in the garage (doing nothing). Earth rod to garage in 6mm Multistrand and connected.

Twin and Earth IR Line/Neu +999, Neu/Earth +999 and Line/Earth 0.01. Removed Earth from main Cu and RCBO stable and live.

Zs 46.6 using just the Earth Rod, connected at the sub board in the garage. RCBO test sequence all clear too. Plugged in Freezer and RCBO tripped. Re-set RCBO and the Freezer is now stable.

2 Questions

Do you connect the CPC on the T/E at the MAIN Db when using RCBO's to supply a garage PME supply?
Any ideas why the freezer tripped the RCBO initially when plugged in for the first time? Leakage currents? RCBO ramp test was 21.0mA

Cheers

Is the garage separated from the house? If so hopefully the T&E is run in some form of conduit? I assume there was a reason originally for them not to want to export the PME - Metal structure? Pipes?

The earth would be connected at the MAIN db to protect the cable (as armour would be if it was SWA) . If the L-E reading was 0.01 on the T&E with it disconnected at both ends then there is damage to the cable which would need to be found.

 

[westward10]

You do not export PME you are utilising a cpc derived from a PME earthing system.

 

[Dartlec]

You do not export PME you are utilising a cpc derived from a PME earthing system.

Noted - I was going from the terminology in an article I read a little while ago from Stroma in PE

 

[pc1966]

If the L-E reading was 0.01 on the T&E with it disconnected at both ends then there is damage to the cable which would need to be found.

I was wondering about that. Sounds like a damaged cable, nail through it, or crushed somewhere

 

[suffolkspark]

100% a damaged cable L-E, why are we all here?

 

[TripleD]

The fault is obvious. And correct cable needs to be replaced.

The practice however and installation method is what has me. No pipe work in the garage. Garage door is conductive as it the frame work. less than 1m from supply cable too.

The earth being connected at the DB end is literally to protect the cable. The question is guess is more around exporting earth on PME/TNC-S systems. And what people are generally doing.

Cant speak for the cable route, but it looks to be plastered in to the wall, run under floors and finally buried directly in the ground inside 32mm rigid duct.

What would you guys/gals do in terms of replacement? Cant drill out of the wall the DB is on due to cabling everywhere concealed behind plaster. Would you take a supply from the Ring by using the outside wall, run in SWA 2.5mm and connect directly to the socket outlets? Then fuse spur the lights with 3a and take the existing rod to an MET? connect this up and restore the supply?

Would you keep the armouring earthed along with the CPC both ends if taken from the ring directly to the sockets? Or just the ring end and abandon it at the garage?

Garage is not attached to the house. I've read some keep the earth connected if its PME and some DON'T?

 

[Dartlec]

Is there much of a load in the garage? Or just a socket and a light?

Only issue I could see with taking a feed from a ring would be the added risk of nuisance tripping the ring. (assuming loading is fine). but a well installed SWA should minimise that. I wouldn't have thought you'd need to go up to 4mm, voltage drop calcs etc. allowing - not like you can load it above the connection to the ring anyway. You'd only need 2 core too assuming the garage is staying TT.

As you say the earth connection for the SWA at the house end is to give fault protection for the cable run.

According to Stroma,

At the remote building, the SWA cable should be terminated at the distribution board or consumer unit using a plastic stuffing gland and not the usual metal SWA gland. The SWA armour should be cut back to under the cable sheath inside the plastic gland so it can’t be touched. ​

Though you could also use a Wiska box adjacent to the CU I guess and then feed just L & N from the supply to the board, along with the E from rod.

Then any extraneous parts in the garage bonded back to the rod MET. I'm assuming the garage sub board already has separate RCD protection as needed?

If your question is related to using the PME for the garage, then it's a risk assessment as to whether it is safe to do so. The presence of anything extraneous in the garage would probably preclude it anyway, unless you were running 10mm SWA to allow for the cpc to also function as a bonding conductor.

Only other option I can think if you have the time and it's feasible is try to work out where the fault might be and then replace that section of cable if it's accessible. In theory the resistance Line-Earth might let you work out how far along the cable it is - Dave Savery did a video on that fairly recently and was able to locate the fault to within 50cm or so.

If the fault occured suddenly they then might be able to work out what's changed - something screwed to the wall perhaps, or similar.