Supply from neighbour's house and TT issues...

[WhydidIstart]

Hi, I think I have a TT supply sourced from a neighbour’s supply next door (terraced house built in 50’s). Although it looks like the neighbour has a TNS or TNCS supply, for some reason only a Line and Neutral are fed through to my supply, no Earth. Does anyone know any reason why no Earth may have been brought through when one is available next door and why I seem to have a TT supply rather than TNCS?

There’s a single 30mA RCD before my consumer unit and I get nuisance tripping as you might expect – say about once a month. This is pretty bad as it means no fridge, freezer, all clocks reset + burglar alarm gets unhappy at the end of the day when the backup battery gets flat.

I’m 98% sure it’s nuisance tripping rather than a fault, but it’s hard to identify as there’s only 1 RCD for the whole installation and it always resets first time without tripping again.

Do you have any suggestions for what I could get done to make the installation more robust?

I was thinking something along the lines of a main 300mA type S RCD to replace the 30mA one for discrimination and then a new consumer unit with 30mA RCDs on each side.

Could the main RCD be taken out altogether as long as every circuit was 30mA RCD protected in the consumer unit, or even better individual RCBOs for everything?

Thanks for your thoughts on this.

 

[Taylortwocities]

The supply is provided by the DNO (supply company).
It will have originally been TT. You should have an earth spike for your property.
You neighbour may have had his upgraded to TNC-S but you would need to ask the DNO if it is available (for you) and they will need to come and make some changes.

You will also need some changes to your installation as the earthing will not be adequate, or configured, for TNC-S..

A 30mA RCD at the front of the installations is bad, for the reasons that you have detailed.

The best way would be to have an RCBO per circuit - this may be expensive at ~£30 per circuit.
Or you could have the CU changed for a dual RCD CU.
In both of those instances you do not need andother "front end" RCD.

A 300mA RCD is not designed for use in the way you describe. It would be a 100mA S type but, as I have said, not required if the board is changed.

You say

I’m 98% sure it’s nuisance tripping rather than a fault, but it’s hard to identify

. It is tripping because of a fault and should be investigated. The problem may be something like the immersion heater, or an appliance so it needs checking by an electrician who is qualified in inspection & testing.
A lot of computer equipment can also cause RCDs to trip.

 

[WhydidIstart]

Great, thanks for your reply. It sounds like the best option is to have the existing 30mA RCD removed and have a CU fitted with an RCBO on each circuit. The house is only small so the cost is worth it to not have everything trip at once and it will make finding any fault much easier!

I wondered if it was ok to remove the main RCD altogether for two reasons: Each RCD would potentially let upto 30mA into the Earth which would mean quite a high theoretical current that would flow into Earth. The second reason would be to prevent someone fitting a new MCB (not an RCBO) into the CU and hence risking an Earth fault not being detected due to the TT Earthing. Obviously that should never happen and good to know the all RCBO solution is ok.

I appreciate I could have a fault, but let me explain more and let me know what you think:

I have isolated sections of the house and none of them cause a trip when they are connected on their own, however only when everything is on together do I sometimes get tripping.

I have quite a few things that wouldn’t have been around in the 50s that are can be high leakage: 3 computers, inverter microwave oven, several pyrobar heaters, immersion heater, fluorescent lighting in the garage and more in a shed + all the usual electronics stuff in a house. If the above things are all contributing a mA or two each, then I figure more than 20mA is being leaked into Earth in perfectly normal operation. The microwave alone leaks 7mA when running. The RCD is also at least 20 years old, so I figure it's constantly just on the edge of tripping and something like the immersion, washing machine or fridge switching on is enough to take it over the edge... What do you think?

 

[flameport]

The front-end RCD can be removed if all RCBOs are used.
Yes, someone could fit an MCB in the future which would cause danger, but they could just as easily do many other things which would be just as bad, if not worse.
Even with a 100mA RCD, it's still a single point of failure, so not desirable.

The microwave alone leaks 7mA when running.

Not sure how you measured that, but if correct, that seems very high. Maybe time to get a new microwave?

Many appliances will have some leakage, and these can be enough to trip the RCD. This is why installations should be divided into 2 or more sections each with a separate RCD (or even better, all RCBOs fitted).

 

[Taylortwocities]

Three is no point leaving a 30mA at the front end if you fit all RCBOs.
You will have the same situation as now.
10mA on three circuits will all add up and the front end RCD would trip.

Also, if you got a real earth fault it would be anybody's guess as to if it would be the RCBO or the front end RCD thjat will trip. It will depend on the trip time and the point on the sinewave.

If you really want belt and braces you could put a 100mA TIME DELAYED RCD at the front end of all of those RCBOs but its pointless - and will cost you another £80+.

An RCD's trip time doesnt depend on its age. It may deteriorate if its not operated regularly. An RCD tester will tell you if it is in spec. But it is going, so there's no point.

 

[Adam_151]

Not sure I like the idea of SP RCBOs on a TT system...

 

[securespark]

How about a 30mA S type on the tails, then 10mA RCBO's?

 

[Taylortwocities]

30mA S type. Hmm,

I did see a Protek one once but it was 4-pole. Never did see a 2 pole one - do you know of a maker?

 

[securespark]

Think they're special order.

 

[flameport]

Hager make them, but only as an adjustable model (both time delay and current can be adjusted).
They make 10mA RCBOs as well.

 

[WhydidIstart]

Hi, thanks for all the info.

I agree 7mA for the microwave seems high, but it’s an inverter type which enables it to actually operate at say half power rather than just cycling on and off like a normal one. Doesn’t seem to make the food any better though, lol. I think inverters are generally noisy so tend to have big filters and hence the high leakage. Measured with an Earth leakage clamp meter I borrowed + cable that allows just the Earth to be measured.

So the solution looks like get rid of the existing main RCD and get a new CU fitted with all RCBOs.

Adam – why are you not happy with SP RCBOs, it is something to do with a N-E fault causing them to trip , but not disconnecting the N so the fault would remain?

 

[Adam_151]

Adam – why are you not happy with SP RCBOs, it is something to do with a N-E fault causing them to trip , but not disconnecting the N so the fault would remain?

Thats exactly it, you've now got your earth tied to your neutral in your installation, and as TT is generally installed where there is something that makes TNCS either unavailable or a poor choice....