Faulty RCD, electrical board or unknown

[Michael2021]

Hi

Hoping to get some insight from people, as I have had two electricians check the fault, to no avail.

The most obvious fault is the RCD is tripping.

It trips randomly. I have tried a process of elimination and it is not a faulty item.

It will trip when cooker is turned on, when washing machine is powered, when kettle is turned on, shower, nespresso machine. All are on separate MCB's.

It is not tripping for certain items, tvs, sky box, fridge, outdoor lights.

Although the RCD is tripping, the MCB that is linked to the item that is causing the trip, is not.

Had my cousin have a look, who is an electrician for 20 years. Deduced (and hoped) it must be a faulty RCD, as everything visually seemed fine and went through wiring at the box. However he had limited tools with him and no RCD to replace.

I called another electrician I know, who had a spare RCD handy. Replaced, however the new RCD tripped. Went through the above with him and was unsure what the cause is. It was late last night so sent him away and is returning today.

Could people perhaps give some ideas, for me to throw at him.

He did try his multimeter on RCD/MCB, this caused the RCD to trip again.

I had a read through some threads, some mention earth leakage?

Thanks
Michael

 

[newfutile]

You need an electrician with an earth leakage meter ,this will tell you what circuit is leaking how many milli amps , failing that insulation resistance should give you a fair idea of where the fault lies.

 

[James]

Classic symptoms of a N to E short (fault)

this could be on any circuit, not necessarily fed from the RCD.

common locations for this type of fault are
oven, grill, hob.
outside lights
garden lighting
any new pictures or shelves etc. Been put up?

 

[timhoward]

If you have an RCD tripping and the actions that trigger it is on different circuits (you mention cooker and shower) there is a good chance it's a perpetual Neutral-Earth fault with either a single appliance/accessory or the fixed wiring.
Insulation Resistance testing is the way forwards.

 

[Michael2021]

Classic symptoms of a N to E short (fault)

this could be on any circuit, not necessarily fed from the RCD.

common locations for this type of fault are
oven, grill, hob.
outside lights
garden lighting
any new pictures or shelves etc. Been put up?

Any reasons why this would happen suddenly?

No, nothing new in terms of pictures, etc

 

[James]

It will become apparent when the spark finds the fault.
water ingress, physical damage, rust or wear and tare are often the cause.

 

[Michael2021]

If you have an RCD tripping and the actions that trigger it is on different circuits (you mention cooker and shower) there is a good chance it's a perpetual Neutral-Earth fault with either a single appliance/accessory or the fixed wiring.
Insulation Resistance testing is the way forwards.

Ill pass this onto the electrician. I did mention the shower/cooker but fault will still occur if the mcb's are turned off for these applicance and occurs for different items on different mcbs.

 

[timhoward]

Ill pass this onto the electrician. I did mention the shower/cooker but fault will still occur if the mcb's are turned off for these applicance and occurs for different items on different mcbs.

That is the really difficult thing about finding Neutral to Earth faults. The MCB's only typically isolate the live, the neutrals are all joined together for the rcd protected circuits. So turning off a circuit doesn't remove the possibility that the circuit has a Neutral to Earth fault.

Here are two quite bad drawings done on my phone (I'm waiting to gain access to a site to start work!)
Two loads, the MCB for load 1 is turned on, the MCB for load 2 is turned off, no faults.

Then with a Neutral to Earth fault on Load2 which still has the MCB turned off, you'll see some of the N current from Load 1 finds a path through the fault. The RCD notices and says "some electricity has gone the wrong way" and trips in case it's path is through a person.

A decent electrician can use standard test equipment to identify the presence of the fault and diagnose further from there.
(EDIT both MCBs are supposed to be joined with a red line at the bottom)

 

[Michael2021]

That is the really difficult thing about finding Neutral to Earth faults. The MCB's only typically isolate the live, the neutrals are all joined together for the rcd protected circuits. So turning off a circuit doesn't remove the possibility that the circuit has a Neutral to Earth fault.

Here are two quite bad drawings done on my phone (I'm waiting to gain access to a site to start work!)
Two loads, the MCB for load 1 is turned on, the MCB for load 2 is turned off, no faults.

View attachment 93300
Then with a Neutral to Earth fault on Load2 which still has the MCB turned off, you'll see some of the N current from Load 1 finds a path through the fault. The RCD notices and says "some electricity has gone the wrong way" and trips in case it's path is through a person.
View attachment 93302
A decent electrician can use standard test equipment to identify the presence of the fault and diagnose further from there.
(EDIT both MCBs are supposed to be joined with a red line at the bottom)

Thanks Tim, presume he'll find the fault today.