RCD Nuisance tripping

[rabenson]

Hi,

My house is supplied by overhead cables so has a combined main switch/ RCD fitted. This has been trouble free for around 10 years but in the last couple of months has taken to tripping for no good reason. There is no pattern to the tripping, it can happen anytime day or night with no correlation to which appliances are running in the house. It always resets first time withouth having to switch off any of the circuits. It can go for a couple of weeks without tripping or trip as often as twice a day

I have had the Electricity supply company out, who checked the installation and found no fault other than a volt drop of 10v compared to my next door neighbour (who is further from the sub than me!). A followup check by a line team found no fault....

I've gone down the route of unplugging appliances but to no avail - it still trips. A visual inspection of sockets and swithches showed no loose connections.

So, it's a question of where do I go from here. I'm not an electrician and would value some advice from those more knowledgeable on the forum. Is it likely that it is my RCD that is failing? Or is it more likely that I'm getting spikes from outside that are causing the problem?

It occurs to me that replacing the existing 30mA RCD with a 100mA device would both rule out a faulty RCD and make the installation less sensitive to external spikes. (I have a socket in the garage to which I could fit another 30mA RCD for lawnmower protection.)

Any thoughts? does this sound sensible or is there another, more obvious solution that I've missed. Any advice welcomed!

Cheers

Ron

 

[Lectrician]

Just because you have an over head supply does not mean you have a TT earthing arrangement, and therfore a requirement for an RCD as the main switch. It is worth checking your supply arrangements.

A 30mA RCD as a main switch is never a good idea, and is IMO a stupid way of providing RCD protection to a TT system. A main 100mA type S RCD should be installed as the main switch, and then a 30mA split for sockets. Ideally, you want an earthing system other than a TT, and then do away with the main RCD, and just leave 30mA protection for the sockets.

It is fairly rare to find external influences tripping an RCD. It is possible for this to happen, expecially on TN-S systems, but generally you still need a fault in your installation to work with the external influence to cause a trip.

You need to get a spark in to do an insulation test to each circuit individually, after first removing all fuses and all neutrals from inside the Consumer Unit. You can then track down the fault to a circuit. And then split this circuit down until you find the fault.

Commonly, tripping that has no set pattern is due to a neutral to earth fault of some impedance - The more load you put on, the quicker the RCD will trip, and it doesn't matter what load, or what circuit this current is coming from.

It could be the RCD, and to test this you need a sparky to do a RAMP test - this isn't a test that is hugely accurate TBH though. Your RCD should not trip when subjected to a half current test. It may if you already have a fault though, so make sure the spark tests the RCD with all load disconnected.

Any chance of a photo of your CU/RCD/Incommer?

 

[rabenson]

Hi Lectrician,

Thanks for your reply! As I said earlier, I'm not an electrician myself so apologies if I'm not familiar with all of the technical terms! When I bought the house, the previous occupants said that there was a requirement for an RCD. There is a separate earth cable running from an earthing rod outside to the main board and what apears to be a twin core supply cable.

http://i179.photobucket.com/albums/w282/rabenson_photos/IMG_0381.jpg

The installion is as below - sorry for the dodgy photo quality, not easy to get a camera in!


http://i179.photobucket.com/albums/w282/rabenson_photos/IMG_0378.jpg

http://i179.photobucket.com/albums/w282/rabenson_photos/IMG_0377.jpg

http://i179.photobucket.com/albums/w282/rabenson_photos/IMG_0380.jpg

Thanks again for your help!

Ron

 

[festa]

Hi rab...
When the electricity board came round did they tell you it was a T.T earthing system? If they did then your incomer must be of an RCCB type (as you have now) but your 30mA one will cause nuisance tripping.
As Lectrician says tests need to be carried out to see if a fault exists first.
When you call a sparky in ask him/her their opinion but mine like many more on here (maybe not all),is to get a 100mA TIME DELAYED/S-TYPE version to replace your 30mA one.
You will need a split load board,as the sockets in your house must have 30mA protection and its always better to have an electric shower (if you have one) protected by this as well.
These 100mA incomers can be quite expensive but screwfix and TLC do stock them for around £43 plus VAT (80A 100mA TIME DELAYED INCOMER).
If that seems steep then mail me as i can get some of a different make for around £30.

 

[Lectrician]

You are connected as a TT system, hence the requirement for an RCD.

Looking at the cable to the house, as it is two seperate cores, and not a concentric (as the ones is that runs from the insulators to the service head by the meter), then it is very rare to find that PME would be aailable before first changing this drop from the pole to a concentric cable.

However, you do have a sticker on your service head that says a "Protective Earthing Terminal" is available, possibly stuck there in error.

I would always prefer a connection to the elec boards earthing if possible, either TN-S or TN-C-S.

 

[securespark]

Don't change the 30mA for a 100mA!!

IMO, a TT system not protected by a 30mA RCD or one protected by an RCD >30mA is AT RISK!

I have inherited an RCD incomer board on TN-S and never had any nuisance tripping - the only time it failed was when a knife was inserted into a toaster!!

You need to check your RCD is working as it should, then measure current drain on the earth. Unfortunately, for both these you need specialist equipment, so will need to hire a spark.

If your RCD ramp is 15ma or less it is faulty. However if it is fine, then fitting a mA clamp meter to the earth will tell you how much is draining and by simple detective work you can track down the culprit.

 

[Lectrician]

Don't change the 30mA for a 100mA!!

IMO, a TT system not protected by a 30mA RCD or one protected by an RCD >30mA is AT RISK!

Thats purely your opinion though. Shouldn't we advise as to what the regs actually say and advise?

It is a very poor installation that purely has a 30mA RCD as an incommer. Usually a spark who has no clue of earthing arrangements and the requirements of the regs - they just put it in as they know thats the safest bet.

 

[viewer]

I'm not qualified to argue the points with Lectrician and Securespark, but can say this:
I have TT. I have a split board CU. There is a 30mA RCD on the normally protected side and RCBOs on the other side to protect shower and socket circuits. I regularly test using ramp test (just to remind myself how to do tests). All works well and when the electric pan died it tripped. No nuisance tripping in 5 years since this was fitted.
My "abode" was originally an army hut and over the years became a semi-detached bungalow. The adjoining property also has an overhead supply, but has PME TN-C-S.
V

 

[rabenson]

Thanks, Guys!

seems like I'm going to be needing the services of a spark....

Thanks for all your help - at least I've got an idea of where I'm going now!

Cheers,

Ron

 

[securespark]

Don't change the 30mA for a 100mA!!

IMO, a TT system not protected by a 30mA RCD or one protected by an RCD >30mA is AT RISK!

Thats purely your opinion though. Shouldn't we advise as to what the regs actually say and advise?

Am I alone in thinking that a TT install with no RCD protection is AT RISK?

After all,

You are connected as a TT system, hence the requirement for an RCD.

I agree. All TT-fed installs must have RCD protection due to the high Ze.

To replace a 30mA incomer on a TT supply with one of greater value places the user at risk, because the higher-rated RCD no longer offers personal protection.

Surely that is what the regs say? Please feel free contradict me!

As for an RCD incomer, it may be "a very poor installation" IYO, but in the eyes of the regs it is only "NTCS".

You must agree that an installation (especially a TT-fed one) has to be safer with an RCD incomer than no RCD protection at all?

 

[Spark123]

I thought it was OK to use a 100mA RCD providing the 50v (or 25v) rule is met. A 30mA RCD is still required for sockets reasonably expected to supply portable equipment outdoors and the other normal bits.
No RCD protection on a TT is at risk.

 

[Lectrician]

Thats what I was getting at - Ideally on a TT you want 100mA or more (but typically 100mA on a domestic) as the main sw, and the 30mA for the sockets via a split board or RCBO's for the sockets etc.

It sounded like Securespark was saying you must have a 30mA RCD as the main switch, and not a 100mA. Thats what I was saying.

 

[Adam_151]

Am I alone in thinking that a TT install with no RCD protection is AT RISK?

In the real world with the values you'll get from an electrode, then no, you're clearly right about it being "at risk" (is that a term you have stolen from the gas boys, ooi?)

Interesting enough IIRC the regs don't actually right out say you need an RCD, they state it is the prefered method, the other being the tradional ADS through overcurrent devices which you can never use because you'd have more chance of a BJ from the pope than getting the impedance on the electrode low enough... (not to mention being sure it will stay low...)

 

[securespark]

(is that a term you have stolen from the gas boys, ooi?)

Yes, I confess, Guv!

I find it a bit easier to explain using this term. Most (but not all) people I find are familiar with it because invariably they have had their boiler serviced for many years & at some point the phrase "AT RISK" has been mentioned.

It sounded like Securespark was saying you must have a 30mA RCD as the main switch, and not a 100mA. Thats what I was saying.

No, just that you should have 30mA for personal protection.