TT systems and a17th ed board

[alymart]

i am in the process of of rewiring a house which is part of a TT system, and i was going to put a 17th ed board in with duel 30ma rcds, and i was not sure if i needed to change the main switch for a 100ma rcd as all circuits will be covered by a 30ma rcd, with no spare ways between M/S and first RCD.

 

[rich.250]

Aslong as all circuits are 100mA protected minimum. Youll be fine.
Obviously 30mA needed for additional protection. Sockets, bathrooms etc

 

[Des 56]

If its all insulated,you need not
If its metal clad,you need to protect the tails,either with an rcd or the use of improved means of entry for the tails

 

[alymart]

thank you, i thought as much, but reading the regs to makes you doubt your self with the way they word thing

 

[D80]

A 100mA Time Delayed RCD front end would help. RCD's aren't the most reliable of things and its likely its the only thing that will see the installation meet the disconnection times.

As Des says though, if its Class 2 board then by the regs you don't really need to.

 

[alymart]

yeah its just a standard plastic enclosure duel rcd board, just hadnt done a TT installation since the 17th came in, as always used to put in 100ma rcd front end but was unsure if it was ok to use the duel rcd board and still have the M/S rather then the 100ma main switch, as there are no circuits or room for circuits before the first rcd

 

[Engineer54]

I think people are crazy installing these 17th dual RCD CU's, on TT systems. They can be nothing but trouble even in TN systems. It's got to be far, far better to have an all RCBO CU in such circumstances where TT systems are involved.

Even then, i'd be inclined to back them up with an S type 100ma RCD front end main switch, with anything like the usual UK 200 ohm Ra value on the Rod!! lol!!!

 

[Guitarist]

Agree E54. Trouble is, one goes in advising the customer that the setup you mention is the safest, least disruptive system for their needs. Then another contractor comes in with a price for a 17th edition board for a lot less. Customer thinks "wow, that first electrician was trying to rip me off".

 

[rich.250]

Agree E54. Trouble is, one goes in advising the customer that the setup you mention is the safest, least disruptive system for their needs. Then another contractor comes in with a price for a 17th edition board for a lot less. Customer thinks "wow, that first electrician was trying to rip me off".

I always price for both, and explain how much easier to use and more convenient rcbo's are.
Haven't fitted a dual Rcd for over a year. And then you can't be made to look bad

 

[robsparky1975]

Exactly,someone will come in and quote £150 to do the job(does anyone remember the thread?)

 

[PC Electrics]

I think people are crazy installing these 17th dual RCD CU's, on TT systems. They can be nothing but trouble even in TN systems. It's got to be far, far better to have an all RCBO CU in such circumstances where TT systems are involved.

Even then, i'd be inclined to back them up with an S type 100ma RCD front end main switch, with anything like the usual UK 200 ohm Ra value on the Rod!! lol!!!

I think I'll take you with me whenever I go to quote a cu change. Your powers of persuasion must be legendary.

All rcbo board and a 100 quid s-type. I'd never get any work if I went in with that!

 

[keniff]

I think people are crazy installing these 17th dual RCD CU's, on TT systems. They can be nothing but trouble even in TN systems. It's got to be far, far better to have an all RCBO CU in such circumstances where TT systems are involved.

Even then, i'd be inclined to back them up with an S type 100ma RCD front end main switch, with anything like the usual UK 200 ohm Ra value on the Rod!! lol!!!

Completely agree here

 

[Engineer54]

I think I'll take you with me whenever I go to quote a cu change. Your powers of persuasion must be legendary.

All rcbo board and a 100 quid s-type. I'd never get any work if I went in with that!

Exactly,someone will come in and quote £150 to do the job (does anyone remember the thread?)

Get a decent Ra value on the TT system and you can do away with the S type!!

At the end of the day, the customer is only going to get what they pay for!! And we all know, they ain't gonna be getting much for there 150 quid!! That's down to you, to explain to these customers that they could well be paying even more at a later date, to put right, what will be an almost DIY installation, by an under qualified electrician!!

Let's remind everyone here, we are talking about a TT based earthing system in this thread, where in all likelihood the TT system is going to be based on numbty Ra levels, so the better the protection is, the safer the homeowner is going to be!! I'm pretty sure if you check around, you can make up an all RCBO for not that much more than a good dual RCD 17th CU. Depends on how many circuits the installation has....