Nuisance RCD tripping questions

[DUB88]

I have had a problem with a very random tripping of the RCD on my CU. I have isolated all the appliances over time and can't narrow it down. There is no pattern to the tripping, it can happen in the day, in the night, with appliances running or turned off, rain, dry, etc etc.. can't tie it down to a specific pattern.
So I called an electrician. He said the wiring checked out and that the RCD needs changing at a cost of £140. The RCD is a 80A with a 30mA differential. I asked what it was tripping at when he did his ramp check and he said 23mA which he said is low. My understanding is they should trip before the 30mA so first question, is this 23mA low and should I get the RCD replaced?
I wasn't totally satisfied with his price so I decided to take the cover off the CU and have a look for a part number of the RCD just to price it up. I noticed one of the earths for the downstairs lights MCB loose so I tightened it up. I know the RCD will trip when there is a difference in current between the Live and the Neutral but could the loose earth on a MCB cause the RCD to trip without the MCB tripping?
Any help would be greatly appreciated...[/i][/u]

 

[Chri5]

The light earth isn't likely to be the problem.

What make is the rcd in question, can you find it here?

http://www.tlc-direct.co.uk/Main_Index/Consumer_Units_Index/index.html

£140 for a swap out is a bit high, most rcds are £30-£40.


Some home equipment is earth leaky, old electric ovens, fridges, freezers and pc power supplies can all leak a few ma and the sum of the leak could trip the rcd.

Do you have many of the above?

 

[DUB88]

Thanks for that. I couldn't find it on that link but it is a Legrand 086 31, 80A 30mA
The oven and the Fridge freezer are less than 9 months old but tripping was happening before they were installed.
This has started with the new extension but the wiring has checked out.
It can trip at night time when there is very little demand infact it even tripped one weekend when we were away when only the alarm and fridge freezer were on. I have isolated the new fridge freezer but it can still trip.
Alarm people are coming tomorrow to service the alarm so will ask them to check it through.

 

[ricicle]

23mA isn't unusually low for a 30mA RCD ramp test.

Another possibly is a trapped cable that gets squashed as you walk over floorboards. Random drip in a boiler or shower.

 

[17thman]

Chris5 is correct but for your info, the 30mA rcd should disonnect when the earth leakage rises to 30mA. So the 23mA ramp test result shows that it is a bit low. The sum of leakages accross connected circuits/equpment can reach 23ma more easily than 30mA

 

[DUB88]

Chris5 is correct but for your info, the 30mA rcd should disonnect when the earth leakage rises to 30mA. So the 23mA ramp test result shows that it is a bit low. The sum of leakages accross connected circuits/equpment can reach 23ma more easily than 30mA

Will a new RCD definitely test higher then?

 

[ricicle]

Every 30mA RCD I have ever ramp tested goes between 20 - 25 mA.

 

[DUB88]

Every 30mA RCD I have ever ramp tested goes between 20 - 25 mA.

Ok if the RCD and the wiring are fine and no one appliance is leaking excessive current I'm going to really struggle getting to the bottom of it..
When the RCD trips it resets fine so for me (obviously not an electrician) the fault that tripped it has gone as the RCD remains latched.
Increasing the RCD rating to a 40mA ( just an example) could solve the problem of nuisance tripping but obviously leaves a serious safety issue, as its 30mA for a reason.
Getting a 30mA auto-resetting RCD again solves the nuisance tripping but raises other questions!
Reducing the amount of appliances plugged in at any one time seems logical however as I mentioned before it has tripped with just a fridge freezer and alarm on. The fridge as mentioned is new and this problem started before.
Now do these split CU's come with two 80A 30mA RCD's? This should in effect spread the current leakage across two RCD's ( just my understanding please set me straight). If this would work is there any other test I can get done bar PAT testing every appliance in the house as the cost of a new CU ain't cheap?

 

[flameport]

No such thing as a 40mA RCD, the next value after 30 is 100mA, and that won't be suitable or comply with regulations.
Auto reset RCDs are not permitted in domestic installations.

Dual RCD consumer units are available, but your previous description implies an intermittent fault somewhere, which no amount of rearrangement of RCDs will fix.

As this problem appeared with your new extension, that would be the first place to look for faults.
Someone saying the 'wiring checked out' doesn't really mean anything - what tests were done and what were the results?

 

[DUB88]

No such thing as a 40mA RCD, the next value after 30 is 100mA, and that won't be suitable or comply with regulations.
Auto reset RCDs are not permitted in domestic installations.

Dual RCD consumer units are available, but your previous description implies an intermittent fault somewhere, which no amount of rearrangement of RCDs will fix.

As this problem appeared with your new extension, that would be the first place to look for faults.
Someone saying the 'wiring checked out' doesn't really mean anything - what tests were done and what were the results?

Thanks. I'm making an assumption ( probably a dangerous one) that the electrician who came today checked out the wiring correctly for this type of problem. Insulation resistance tests etc...

 

[theprinceofdarkness]

When I lived in South Hampshire, we had phantom trips a few times a year at about tea time on Fridays. It was tied up with the local industrial estate closing down for the weekend and the CEGB automatic tap adjusters putting spikes on the mains. Changing the RCD to 100mA cured this problem. Later on we had a metal chandelier with some hydroscopic chemical in ONE arm, this caused moisture to accumulate on the back of bayonet connector with resulting RCD trips. Why only one fitting? What was the chemical? Apart from this fault it was a very nice looking jobbie. From memory it was from British Home Stores, American Bar range.
Frank

 

[TonOfSand]

I had a similar problem which turned out to be the washing machine in the garage. Now I switch off at plug. More recently my tumble dryer tripped the RCD (all circuits on 30mA). I directed the vents well away from appliance & that cured the problem. In these cases it would be so difficult to determine the cause, as the tripping was so random & unplugging & replugging to determine the circuit wouldn't have worked for same reason. The point being: check out your appliances in damp conditions.

 

[EFLImpudence]

Changing the RCD to 100mA cured this problem.

So would removing all RCDs.

N.B. Safety devices should not be removed to stop them working.

 

[EFLImpudence]

Now I switch off at plug.

That won't make any difference with N to E faults.

Edit - unless they are double pole switched sockets.

 

[thomo27]

nuisance tripping is a pain in the arse more so when the RCD takes out the whole board..Its at times like these when individual RCBO's come in handy as this will pinpoint the circuit..A split load board would narrow it down...