17th Edition Board on a TT system

[danny7299]

Before the 17th edition , we used to put a 100ma Timed delayed RCD as the main switch to protect the lighting, cookers etc and a 30ma on the sockets.

Now the 17th edition is out, 30ma on pretty much everything, my thinking is you dont need the 100ma Timed deley?

ohh...in a domestic installation of course

Is that correct?

many thanks

:Y

 

[NozSpark]

If you put a twin (30mA) RCD CU in then that is complies for a TT install as all circuits will be RCD protected.

 

[danny7299]

thats what i thought,,, but its always nice to check.

I hate TT systems there **** ! hehe

many thanks

 

[sarni31]

My understanding is the 100ma time delay rcd provides discrimination. ?:|

However if you have every curcuit on its own 30ma rcbo ......?

Guiness Drink

 

[NozSpark]

My understanding is the 100ma time delay rcd provides discrimination. ?:| However if you have every curcuit on its own 30ma rcbo ......?

Guiness Drink

It does when you have the 100A/100mA TD RCD as a main switch, but when you have a main isolator feeding two seperate RCDs then there are no discrimination issues..

There are some issues with single pole RCDs and TT installations.

TT installations are great BTW

 

[sarni31]

It does when you have the 100A/100mA TD RCD as a main switch, but when you have a main isolator feeding two seperate RCDs then there are no discrimination issues..There are some issues with single pole RCDs and TT installations.

TT installations are great BTW

true... but if you want to reduce the inconvienience factor even more then RCBOs. As 30ma and less rcds are fitted as 'additional protection' to the overcurrent devices where as the 100A/100ma rcd protects the installation surely 100ma RCDs need still to be fitted ? ?:| ? :|

Guiness Drink

 

[NozSpark]

If you have 30mA RCD protection on all the circuits then you do not need a 100A/100mA TD RCD main switch..

16th regs said that all circuits need 100mA RCD protection to protect building/livestock & socket circuits need 30mA protection... now with 17th pretty much all domestic circuits need to have 30mA protection. There is a reg that says that any fault protective devices should be double pole, so single pole RCBO's are a no no

 

[Evans Electric]

Interesting that Danny hates TT jobs and Nozzo loves them. I too hate them , reason is we have come across a lot of TT s where the rod and /or the cable to it has been removed by builders , gardeners etc .

Also in North Birmingham we have had problems getting decent readings from our rods, last one we put four rods with extensions in a front garden and still had carp readings.

Ended up on the cable sheath at 14 ohms .

Doing most of our work in the built up areas of the West Midlands it is only of late that we are having to do TT s as the network earthing is breaking down at the joints. Also doing mostly industrial/commercial an earth terminal is always provided with the supply.

I appreciate that those of you in rural areas are well used to them .

 

[Evans Electric]

Danny the more I look at your avatar the more I see ET .

 

[danny7299]

ET

I see what you mean

 

[NozSpark]

Around here you can get 15 to 35 ohms with a single rod no problem..

I think the main benefit with a TT is that you are not relying in an old network for your earth connection..

Say you have a TNS supply (lead sheath) and the sheath rots... you now have a TT, but you don't know it's TT!!

 

[Andy™]

Say you have a TNS supply (lead sheath) and the sheath rots... you now have a TT, but you don't know it's TT!!

yes, but you end up with a high Ze, unlike the non-existent earth if there a fault with PME

 

[a1]

Personally, I like TT's - and it is suprising how many TT systems we have in my neck of the woods.

Considering I ain't in a rural area. :|

 

[Andy™]

TT can be better for domestic, but for commercial, its best avoided. saves the hassle of RCD incomers (since 99% of TPNE board are metal). and RCBOing a TPNE board isnt going to be cheap!).

round here, its mostly TNS, newer are TNCS, but there are still a large number of TT, even in the built up area's

 

[NozSpark]

yes, but you end up with a high Ze, unlike the non-existent earth if there a fault with PME

Ah but if you lose the Neutral with a PME installation you lose your suppliers Neutral and earth,,,, this will then make your PEBs your Neutral and earth path - not good, especially if your services enter your property in plastic!!! :O:O:O:O:O

 

[jl-heating]

A TT supply needs a 100 ma rcd main switch as well as the 2 x 30ma rcd on the board.

we work to local council spec and this is on the spec.

we generally fit up to 10 boards a week on tt supplies and all our upgrades have the 100ma main switch fitted.

we have even spoke to the wylex rep regarding supplying them already on the board and he is getting back to us with a price .

 

[Andy™]

A TT supply needs a 100 ma rcd main switch as well as the 2 x 30ma rcd on the board.we work to local council spec and this is on the spec.

100mA RCD main switch is not required by 7671. maybe for that council, but everyone else doesnt have to do

what exactly are tey going to gain by RCD'd the internal wiring between RCD (/isolator) and RCD

 

[jl-heating]

there are reasons to fit and reasons not to bother i would rather aire on caution and over protect.

i only know we fit them and sometimes its a pain to actually sourse 1 but there you go we sleep at night knowing all bases are covered.

 

[Andy™]

i dont see any point, but then were all aware of council requirements. usually from someone who doesnt know electrics and has second hand info and makes decisions from that

 

[jl-heating]

they also cross bond the boiler on all five pipes along the bottom.