RCBO keeps tripping out to outbuildings

[jamesBen80]

I've just gone out to the garage and found the power was off. The small fuse board in there was all on, but when i check the main board in the house the 45A RCBO that feed the garage was off. I tried to turn it back on and it just trips straight off again, wont stay on. I then went out and turned all the circuits in the garage off and the main switch and tried the RCBO again. Still wont stay on. So I disconnected the cables out from the bottom bottom of the RCBO and now it stays on. so I assumed there is a fault on the cable somewhere. I then disconnected the cable from the board in the garage and checked for shorts between Live, Natural, and earth all fine? so i re connected everything and it still wont stay on. any ideas?

 

[JohnD]

If the main switch in the garage does not isolate the fault, it is upstream of the switch.

Are there any joints or connections between the house and outbuilding?

Did the fault arise after rain?

 

[bernardgreen]

checked for shorts between Live, Natural, and earth all fine?

Did you do any checks Live, Neutral and Earth to ground,
Damage to the cable that allows moisture in the ground to reach one of the cores in the cable will cause the RCBO to trip on Earth leakage but will not show on tests Live-Neutral, Live-CPC or Neutral-CPC ( CPC being the Earth wire in the cable )

 

[jamesBen80]

No its a single cable between them no joints. We did have rain so could be a possibility, but its run through a duct the whole way and the duct looks dry. I've just re checked with the cable disconnected completely at both ends (hanging in free air as opposed to in a screw terminal with the switch off at the garage end) and i get zero ohms Live to Nutural, zero ohms Nutural to earth 55k Ohms live to earth. so this could be the issue. There are 2 inspection chambers along the ducts route I can lift to check for water or vermin activity. I'll do this next. Checking to ground might be difficult as the ends of the cable are not near any exposed ground so I'll need a very long test lead, which i've not got to hand.

 

[JohnD]

You can repurpose an extension lead, if you have a long one. In some cases a metal waterpipe can be used.

As you have a duct, you could pull out the cable for inspection and test.

 

[crystal ball]

What instrument are you using to test the cable, if it is fully disconnected you should have Megohm readings between cores, what sort of cable is it?

 

[jamesBen80]

it's nearly 60m so I'd prefer not to pull it out if I can help it. If I do pull it out I might as well pull a new one in at the same time, as it's going to take ages and at least i should have solved the issue with a new cable. If it is the cable. One thought i had was to wire and extension lead in over the ground just to rule out the RCBO and fuseboard in the garage. If the RCBO stays on i know the fault is with the cable?

 

[jamesBen80]

Hammered a long screwdriver into the wet ground outside and repurposed an extension lead to run back to the garage consumer unit checked between Live, neutral and earth and the ground outside, via the extension lead and screwdriver and I've got infinite resistance i.e. dead short on all 3, So I can only assume the cable and duct has failed and the cable is sitting in water somewhere in the duct creating the short. I also checked the consumer unit the garage against the ground outside and neutral and earth showed grounded to the ground outside but not live so I can only assume there is an earth rod somewhere in the garage perhaps?

 

[jamesBen80]

Just a standard multi meter with manual range selection. the cable is 16mm T&E

 

[JohnD]

infinite resistance i.e. dead short

Infinite resistance is not a dead short.

It is an open circuit; no connection.

 

[JohnD]

neutral and earth showed grounded to the ground outside but not live

This is correct.

Please photograph the incomer, company fuse, meter and house CU and the cables around and between them, especially any that are bare or green and yellow. There should be main bonds to your incoming metallic services, especially the water pipe.

 

[jamesBen80]

sorry my bad English I got it the wrong way round I meant dead short. Water pipe is blue MDPE plastic, and gas is yellow MDPE plastic so no bonds where they come up from the ground when they change to copper they are earth bonded to as are the pipes in the airing cupboard back to the CU in the house. There is a copper earth connector strip next to the meter where the mains cable comes up into the house. this is earthed tagged to the outside of the black mineral insulated mains cable as it goes into the company fuse and also links to the CU, meters and pipework around the house. This was all checked 4 years ago by the supplier when we moved in and had a new CU fitted and the supplier fitted a mains isolator between the meter and CU.

 

[JohnD]

From what you describe of the jnstallation,, and

neutral and earth showed grounded to the ground outside but not live

The earthing is all correct.

The photos would be useful.

 

[winston1]

Water pipe is blue MDPE plastic, and gas is yellow MDPE plastic so no bonds where they come up from the ground when they change to copper they are earth bonded to as are the pipes in the airing cupboard back to the CU in the house.

They don’t need to be bonded if the incommers are plastic.

 

[jamesBen80]

thats what i thought just bridging the plastic tanks and bath etc. and join all the copper back to the main earth point my the meter