TT Supplies & Time Delayed RCD's

[dannyboi2003]

Guys,

When did the regs change to specify that a time delayed 100ma RCD main incomer had to be used on TT supplies in conjuntion with a 30ma RCD for the socket circuits.

I only ask as i've just popped up the road to reset the RCD for an elderly neighbour and her consumer unit has obviously been changed recently and the earthing has also been updated. I'm told this was done by an electrician last year but no time delayed RCD has been used. Its just a standard wylex split load board with 100a main switch and 80a 30ma RCD. The lights and cooker are fed from the main switch side with the sockets being on the RCD side, also one Siemens MCB has been used for one of the socket circuits. The overall CU change looks a bit scruffy as well, i mentioned it wasn't to current standards and would probably need to have the main switch changed but i said i'd double check and let her know.

Has the "spark" messed up or was it just not in the regs to use time delayed RCD's until recently?

Thanks
Dan

 

[nozspark]

Sounds messy and is certainly not to regs for a TT install. All circuits must be RCD protected protected in a TT installation, 100mA for lights, oven etc - 30mA for sockets as per usual. The reason you use a TD RCD is to do with discrimination as if you have a 200mA earth fault n the socket circuit then the 100mA main RCD could trip instead of the 30mA one. This has been in the regs since at least the 2004 amendment and I would think that it has been there even longer than that.

Are you sure that this is a TT and not TN-C-S with a local rod??

If you go down the route of fitting a 100mA TD RCD you may have difficulties as not all manufacturers make them, also the last one I bought cost £70 - £80.

 

[viewer]

Exactly which regs require a time delayed RCD?

 

[nozspark]

BS7671

Circuit arrangement 314-01-02
Requirement for RCD (touch voltage) 413-02-19
Discrimination 531-02-09

Prob missed something there, but someone will fill in the gaps I'm sure

 

[megawatt]

May be cheaper to get the DNO to supply an earth if possible ... Just a thought.

 

[securespark]

If you have sep units, you don't need a TD RCD.

 

[dannyboi2003]

Thanks for the replies guys.

nozspark - yes i'm certain its a TT supply, the mains supply for the street (my house included) is overhead and each house has it own earth rod outside the front door, there is no DNO MET and PME is not available. I wont be doing any work on the CU myself, i merely said i would get back to her with some more information. It was actually her daughter who contacted me this morning asking if i could help as her mother had no power, she arranged to have the CU replaced and the wiring checked last year and said if it wasn't done properly then she would call the electrician back to explain himself, oops what have i started

megawatt - i have enquired about PME myself and was told its not available in our area due to the age and configuration of the network.

Thanks for the replies guys, i'll make a short and brief list for my neighbour and leave the rest up to her.

All the best
Dan

 

[RF Lighting]

It really wants something doing ASAP, even just swapping the main switch for the RCD for the time being. This will still not comply with the regs, but will be a damn sight safer than the current dangerous installation.

As it stands, a fault live to earth will NOT cause the OCPD device to operate due to the very high Ze. (as is normal with a TT supply)

This will mean that under fault conditions, ALL metalwork in the house will rise to mains voltage, and remain live shocking anyone who touches it.

ELCBs (an old incarnation of the RCD) have been installed on TT supplies for years and years.

I think the CU has been swapped out by some clueless buffoon, who wants taking outside and beating with something particually unpleasant.

 

[Spark123]

Agree with Rob, if it is TT then the whole installation should be protected by RCDs. Was an EIC provided to the client when the CU was replaced?

 

[viewer]

Sorry to be prodding the snake, but I wonder if there is a misunderstanding here.
No doubt that the whole installation needs RCD protection (Regulations 413-02-20 and Regulation 412-06).

Time delayed discrimination is suggested to prevent complete loss of power if the single RCD trips. There is no requirement for this - the requirements of Regulation 531-02-09 refer only to situations where RCDs are connected in series. There are thousands of houses in UK with TT systems and a single 30mA RCDs and no problems.
It seems the real problem for the OP is that the cooker and lighting are not protected. I do agree that cookers are prone to leakage and a TD or RCBO may be a very good idea, but to be pragmatic and protect the lady, just get all the circuits on the RCD side of the CU.

 

[NamelessRob]

Isn't it safer to leave the lights unprotected by the RCD? There must be a greater risk of injury by having an elderly person's house suddenly lose all light than from a fault on a lighting circuit?

 

[davy_owen_88]

Isn't it safer to leave the lights unprotected by the RCD? There must be a greater risk of injury by having an elderly person's house suddenly lose all light than from a fault on a lighting circuit?

As it stands, a fault live to earth will NOT cause the OCPD device to operate due to the very high Ze. (as is normal with a TT supply)

This will mean that under fault conditions, ALL metalwork in the house will rise to mains voltage, and remain live shocking anyone who touches it.

Hence the reaso n for a time-delayed RCD. It reduces the chances of the lights going off unless the fault is on the lighting circuit. In those cases it would still be safer for the power to go off than for someone to get an electric shock and die. Also an earth fault on a supply with high Ze can lead to overheating and eventually a fire.Again, I would consider that more dangerous that a lack of lights...

 

[Spark123]

Nope, on a TT system all circuits need to be RCD protected to provide protection against indirect contact. This can be achieved using a number of RCDs to prevent the loss of all lighting and power in the event of one tripping. To completely do away with RCD protection on a TT system will be leaving the whole installation dangerous whith the risk of all connected conductive parts becoming live in the event of a fault on one of the non-RCD circuits.

Beaten by davy!!

 

[megawatt]

Simple ...

www.effectivesoftware.co.uk/DIYNot/Images/Kitchen/mkcu.jpg

 

[Spark123]

I believe Hager do a CU with an isolator and 2 RCDs hence no need for the time delayed one which costs a few £££s