TT earth help

[oharaf]

Sorry John you're dead right!! Off to birch my self good and hard

 

[ElectricDunc]

From what i can gather he has a standard split load, 16th edition style consumer unit with a main switch and a 30mA RCD protecting the socket circuits. Obviously this would mean there is no RCD on the lighting circuits etc and with a high Ze leading to a low PSC on a TT system an earth fault wouldn't even tickle the MCB's so metal work would become live and stay live. Best bet is to stick a 100mA S type RCD in place of your main switch as has already been stated. RCBO's should not be used to replace the MCB's on the non RCD side of the unit as they will not isolate live and neutral which is a requirement on a TT supply unless you can fit double pole RCBO's but i think that will be unlikely.

 

[NotHimAgain]

I was thinking of

i.e. dual RCD covering 2 split loads say 80mA and 63mA

I hope you meant, an 80A 30mA RCD and a 63A 30mA RCD.

Yes, I would formerly have said a 100mA "S" RCD on the incomer, but these days with an insulated CU and all circuits on 30mA RCDs or RCBOs, no need

You should check this out http://www.esc.org.uk/forum/forum_nd_thread.html - look for NDQ14 and the answer.

This advice has recently been changed to specifically mention the need for a CU of all insulated construction after some fine fellow convinced the ESC of the error of their ways. They had made a simple statement like yours but this does not take into account the possibility of a fault between a final circuit cpc and any wiring connected to the upstream side of the RCDs. If you look at some so call 17th Edition boards this could easily occur.

 

[ericmark]

down here we use 30mA RCDs for personal protection.

what make are your 50mA?

Thank you JohnD you are right I made an error it is 30ma much easier when you say why you have quoted I will change the posts.

 

[securespark]

down here we use 30mA RCDs for personal protection.

what make are your 50mA?

You take the Mick, John, but I was called to an installation once. When testing the RCD @ 30mA, it failed. Looking at the unit, it was an RS one rated at















yes you guessed



















50mA!

 

[ericmark]

As far as I am aware only caravans see 708.553.1.13 and marinas 709.531.2 need the neutral switching by the RCD.

And on farms the 100ma RCD is still used for sockets over 32A other than that as far as I am aware an RCD for use with a TT system is selected by calculation.

On a personal preference I would like to see all live conductors switched by the main RCD on a TT supply but I see no requirement for that in regulations except for caravans and marinas.

The marina is an odd one anyway with diodes in earth cables and isolation transformers to stop electrolysis eating away the boat. Lucky my son's marina has wooden jetties but I see good reasons to use gang planks.

 

[Spark123]

All main switches for domestics need to be DP, also all devices for isolation in a TT system need to be DP.

 

[ericmark]

411.3.2.1 stipulates "Line" not "Live".
I have scanned the regs using TT as reference point and have failed to find any regulation which says the protective device must include the neutral. Since you are still allowed to use fuses if you can be sure the Zs is a low enough valve to ensure disconnection on a TT system and you are not permitted to fuse the neutral I would not expect to find any requirement to use double pole devices. But I have made mistakes and look forward to your reply with the regulation number to show where it says double pole must be used.

P.S. If you are referring to 537.2.2.1 then in a split board with RCBO's in the non RCD side it would still have a main isolator which would comply with 537.2.2.1 and single pole RCBO's would be permitted.

 

[Spark123]

537.2.1.1

 

[ericmark]

Provision may be made for isolation of a group of circuits by a common means. if the service conditions allow this.

This would be main incoming isolator.

 

[Spark123]

It could be, but in a TT system needs to be Double Poled.
If the circuit protective device is required to double as an isolator then that also needs to be DP in a TT system.

 

[ericmark]

It could be, but in a TT system needs to be Double Poled.
If the circuit protective device is required to double as an isolator then that also needs to be DP in a TT system.

Agreed but I would think it would be hard to replace the incoming isolator with anything but a double pole RCD.

I was reading it as wanting all RCD protection to switch both line and neutral which is only required with caravans and boats.

 

[Spark123]

Would be better imho, not sure if it is a reg but if you had a N-E fault with no RCD pole protection on the neutral conductor you could end up with pretend TN-CS system.