Discuss TT system rcd question fairly urgent in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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matt456

ok, first things first, happy new year to all members:)

now on to the problem,
i live in a rented house and my landlord has asked me to do some remedial work for him after a recent report came back as unsatisfactory.
1 of the 'snags' was that the db which is installed in a tt system has only 1 rcd? this is 30mA the tester gave it a 4 grade on the test sheet which means non-compliant with the regs but i dont have a copy handy. I have yet to do my 17th but dont remember it on the 16th and have seen hundreds of them fitted like this when i was younger working for a housing company. Are you supposed to fit a 100mA rcd at primary source?
thanks guys
matt
 
ok, first things first, happy new year to all members:)

now on to the problem,
i live in a rented house and my landlord has asked me to do some remedial work for him after a recent report came back as unsatisfactory.
1 of the 'snags' was that the db which is installed in a tt system has only 1 rcd? this is 30mA the tester gave it a 4 grade on the test sheet which means non-compliant with the regs but i dont have a copy handy. I have yet to do my 17th but dont remember it on the 16th and have seen hundreds of them fitted like this when i was younger working for a housing company. Are you supposed to fit a 100mA rcd at primary source?
thanks guys
matt

I think what you may need to either fit a new split load board with main protection via 100mA RCD and a 30mA Rcd for your power cicuits. Or alternatively use of RCBO's for designated circuits.

You current installation circuit arrangement situation doesn't comply with the regs due to the fact your circuits are not seperately controlled i.e. if the RCD trips everything goes off including the lights.

Hope this helps!

Warren
 
cheers warren
had a look just now and there are 2 spare ways and a split neutral bar inside! so think i could conjur a split db up from that if i get a 100mA rcd from the suppliers tomorrow. Didnt think that was a regulation just good practice but there you go! got my new regs coming from amazon as i speak!


thanks for the post office insurance tip saved nearly a 100 quid :)
get you a beer sometime!
matt
 
Hi.

Off the top of my head the ol' red bible says something like - circuits should be arranged in order to minimise disruption in the event of a fault.

The easiest way out of this would be to fit a dual 30mA RCD board as almost every circuit needs 30mA RCD protection these days depending on what it's doing and how the cables are installed etc. When you replace the old board you would put say - downstairs sockets and upstairs lights on one rcd and upstairs sockets and downstairs lights on the other so if one rcd trips then youy dont loose all power in one location.

As Warren said designated circuits i.e smoke alarms should be on their own RCBO to minimise the chance of them tripping off due to the effect of other circuits.

The other way is to fit a board with a 100A main switch and individual RCBO's for each circuit that way only the faulty circuit will trip off. This is the most expensive way though.

Regards.
 
cheers warren
had a look just now and there are 2 spare ways and a split neutral bar inside! so think i could conjur a split db up from that if i get a 100mA rcd from the suppliers tomorrow. Didnt think that was a regulation just good practice but there you go! got my new regs coming from amazon as i speak!


thanks for the post office insurance tip saved nearly a 100 quid :)
get you a beer sometime!
matt

Its case of discrimination between RCD's/RCBO's.

Under the 17th Edition lighting is now protected by RCD's and should be arranged so that the upstairs lighting & downstairs sockets are on one RCD 1. and the downstairs lighting & upstairs sockets are on the other RCD 2.

Idea being that if the upstairs sockets cause the RCD 1 to trip you would still have the upstairs lighting as they are being feed from RCD 2.

Glad to hear you got your insurance cheaper! ;)

Warren
 
Hi.

Off the top of my head the ol' red bible says something like - circuits should be arranged in order to minimise disruption in the event of a fault.

The easiest way out of this would be to fit a dual 30mA RCD board as almost every circuit needs 30mA RCD protection these days depending on what it's doing and how the cables are installed etc. When you replace the old board you would put say - downstairs sockets and upstairs lights on one rcd and upstairs sockets and downstairs lights on the other so if one rcd trips then youy dont loose all power in one location.

As Warren said designated circuits i.e smoke alarms should be on their own RCBO to minimise the chance of them tripping off due to the effect of other circuits.

The other way is to fit a board with a 100A main switch and individual RCBO's for each circuit that way only the faulty circuit will trip off. This is the most expensive way though.

Regards.
not sure if you will get away with main switch and rcbo's as itsTT,I think you need double pole and your rcbo is usually single pole.I think I would go with the 100ma for the lights (not in the bathroom,)cooker,waterheater,boiler,smokedetector.,and if you really wanted to separate the power circuits I think you could fit an rcbo on the 100ma side also. But then again I don,t have 17th edd either,and warren and Len seem to know there stuff so there y'go.:p
 
ALL circuits must now be protected by 30ma RCD/RCBO whatever the earthing arrangements, unless there are circuits with all cables either surface run, in earthed metallic conduit (difficult to prove) or more then 50mm deep in the wall.

However, TT systems do generally throw up more challenges.

But if the service head has a single black incoming cable then its quite possible that your local DNO may be able to provide a PME terminal for you to convert to TN C-S. You must call them first though and they will do a site visit to confirm. Some operators do this service for free, but not all.


You chaps do need to get on your 17th edition courses!!
 
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Sorry Matt, went off on one a bit:D

16th edition and before required socket outlets expected to supply equipment outdoors required 30ma RCD. Lights and others not required.

TT systems required ALL circuits to be protected by RCD as most disconnection times could not be met due to the nature of the systems.

30ma RCD's as a main switch was frowned upon as this could cause danger if the whole CU went out due to earth faults. As lighting had to be RCD protected as well, we were permitted to use a 100ma time delay RCD as the main switch as this would not trip if there were a fault to earth on the 30ma side.

On TT systems now, you could fit a double insulated or class II DB with 100A main switch and dual RCD's as this meets current requirements.
 
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kinell! cheers jason now my heads battered!
so you can buy a split board with separate main switch? as well as 2 rcds or is the primary rcd used as main switch?
i have only been out of house bashing for a couple of years and already im out of date aaaarggghhh!
 
thanks to all who have helped!
i went down the 17th edition style db route
already had the mcbs so just needed the board and was 45 quid with 1 main switch and 2 30mA rcds so pretty reasonable really
thanks again for your time and advice guys
all the best for the new year
matt
 

Reply to TT system rcd question fairly urgent in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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