Garage tripping house

Jeepers_Creepers

The MAIN switch in your garage's CU should isolate both L and N. So, if you turn that switch off, then you have effectively isolated the supply to the garage at that point.

Ie nothing beyond that point - not a light switch, not a socket, not the garage outside light - are now involved, so they can be discounted from your head scratching. (Proviso - relying on the whole caboodle having been wired correctly in the first place).

So, almost certainly the leakage that is tripping your house CU is between the garage CU and the 'fuse' for the garage in the house CU.

Most likely there is a joint in that cable somewhere, and some rain has got in there. The cable under the ground is unlikely to have been damaged, surely?

When you say 'fuse' in the house CU, I suspect you mean 'MCB'? Ie, it is reset by flicking the switch back up?

That MCB itself isn't 'blowing'/tripping at all? It's the MAIN switch - the 100A jobbie with the RCD - that's tripping, yes? That suggests you do not have a short-circuit or anything really bad like that, but just a 'leakage' of power from the Live wire to earth somewhere along that garage wire. One drop of water can do this.

So, assuming what I've surmised about your CUs above is correct, forget looking at your garage wiring beyond the garage CU, but instead physically examine every visible inch of the cable that goes from the garage CU to the house CU.

WHERE does that wire disappear underground? WHERE does it come back up? What type of cable is it - is it normal grey flat T&E, or is it the proper stuff like SWA (round armoured cable)? If the former, does it run in a conduit under the ground - a pipe of some sort? If so, what kind?

And, and you CERTAIN there isn't a junction box either outside, or on the garage wall before it reaches the garage CU?

HoolyNI

Both the mains fuse box with the trip switch and garage mains are at ground level with the leads presumably underneath going down into the ground and we've had no real rain in a week so I don't think water is an issue and there no junction box that am aware of.

The only fault i've found is the dodgy light switch which will be replaced before calling a spark out. All the wiring was done mid 90's and its the normal grey cover with e n & l inside.....with the garage CU switched off totally when I push the 30amp fuse back in at the main board in the house it trips.

Jeepers_Creepers

HoolyNI said:

Both the mains fuse box with the trip switch and garage mains are at ground level with the leads presumably underneath going down into the ground and we've had no real rain in a week so I don't think water is an issue and there no junction box that am aware of.

The only fault i've found is the dodgy light switch which will be replaced before calling a spark out. All the wiring was done mid 90's and its the normal grey cover with e n & l inside.....with the garage CU switched off totally when I push the 30amp fuse back in at the main board in the house it trips.

This is standard grey Twin&E cable? Does it have any protection buried in the ground? Sleeved? Conduit? Or is it buried in earth and soil which is full of stones and rocks and chips and nasty things...

And this dodgy light switch is beyond the garage CU obviously? Obviously worth checking, but if that turns out to be the miscreant, then summat weird is going on.

HoolyNI

Tried a new switch on it but no joy, still tripping. The switch is on the garage CU circuit, as for the cable I presume it's under flags in the yard, as all that was done when the house was built which was before my time here. Have put things back as they were and will get the professionals in tomorrow.

Thanks for your input, but whatever is causing it is beyond me...

Jeepers_Creepers

You've tried everything you can, I think.

The pros will have test equipment that'll instantly tell you if there's a 'leak' in the cable under the flags.

Could you come back and tell us the result, please - it's always good to hear how things pan out?

Cheers :-)

HoolyNI

Just a quick update. Turned out there was a fault in the live lead where it exits the house, about 5 metres of cable was replaced between there and the garage CU. All working now, although my freezer was in the same socket as my router and network plug temporarily and for some reason both stopped working. Had to get my router replaced [foc thankfully] and the network plug was actually tripping the "domestic plug" fuse until I traced it via process of elimination. Could the freezer in that socket have caused all that?

Also never thought to ask the spark but getting back to the garage issue how did the cable stop working at one end but not the other? How come he only had to replace half the cable and  not the entire length?

Section62

HoolyNI said:

Also never thought to ask the spark but getting back to the garage issue how did the cable stop working at one end but not the other? How come he only had to replace half the cable and  not the entire length?

If the cable was the cause of the problem then the most likely fault was the failure of insulation on either (or both) the live or neutral conductors, allowing current to go where it wasn't supposed to.

That could be caused by a manufacturing problem, an installation fault, or else subsequent damage. And there could be one or many insulation failures along the whole length.

If there was only one fault then only that part of the cable needs to be replaced - the rest of it would carry on working fine.

Good reasons to replace the whole length of cable would be if it was -

• cheaper to replace the whole cable than the labour costs of further testing
• impossible to join the cable or making a join would be more expensive than replacing the whole cable
• suspected or known that other failures existed or could soon occur
• the wrong kind of cable in the first place

...otherwise, test, find the approximate location of the fault, replace the affected section, test again, do the paperwork.

Jeepers_Creepers

Thanks for the update, and very pleased it's sorted.

The fault in that garage supply cable was a 'leakage' type, from what I understand from your description. All this requires is for the insulation around the live cable to be marginally breached, so that it 'leaks' some current - it only has to be in the order of 30mA - to ground.

(The RCD in the house CU cleverly compares the current flowing in the live and neutral cables, and checks that they are the same - which they should be. It's only if one of them 'leaks' that shows up as an imbalance between them, and this trips the RCD. It might seem dramatic when the RCD goes 'clunk' and everything goes dark, but the cause can be quite marginal).

So, the section he replaced must have had a slight breach in it - a cracked outer sleeving, some abrasion to the insulation that allowed a tiny bit of damp to reach the live wire, that sort of thing. It almost certainly wouldn't have shown up as anything dramatic like a scorched or melted cable - just a graze. It's a shame he didn't show you the cause - it's always interesting; "Wow - that caused all the hassle? THAT?!"

I don't quite understand your other issue re the freezer and stuff - could you explain in more detail, please?.

coffeehound

HoolyNI said:

... my freezer was in the same socket as my router and network plug temporarily and for some reason both stopped working. Had to get my router replaced [foc thankfully] and the network plug was actually tripping the "domestic plug" fuse until I traced it via process of elimination. Could the freezer in that socket have caused all that?

It could be that when the short-circuit happened, there was a sudden high current flow that was then cut by the RCD.  This can sometimes throw up a voltage spike which might have killed electronic components in the router and smart plug.  (They don't like it up 'em.)

Well done to J_C for correctly predicting the fault

HoolyNI

The cable we pulled out of the ground had no obvious signs of damage though there was water in the conduit it came out of so would only have taken a tiny nick I suppose, the cable itself had an oily residue all over it....as for the freezer, network plugs, I can only assume the freezer drew too much current to that socket and fried something delicate. All sorted now anyway. 

Thanks all for your help and advice.