T T consumer unit upgrade

[Ade1309]

Hi I am installing my first tt system consumer unit I understand that I need a 100 amp time delayed rcd main switch but can I use rcbos in line next to them or Mcbs and does any one know of a lockable board as I need to install it vertically thx

 

[mainline]

Hi I am installing my first tt system consumer unit I understand that I need a 100 amp time delayed rcd main switch but can I use rcbos in line next to them or Mcbs and does any one know of a lockable board as I need to install it vertically thx

A time delayed rcd is not required.

 

[timhoward]

Hi, and welcome to the forum.
Your profile says you have passed the 18th edition exam. I'm making the assumption you have a copy of BS7671?

S-Type RCDs are covered in the Unwanted Tripping section 531.3.2 and the basic point here is you need one if you have another normal RCD downstream, so you don't trip two at once. It isn't a given that you need one at the incomer of a TT system if RCBO's are used to protect all outgoing circuits and the tails are suitably secured. (The on-site guide suggests a tails clamp and clipped to arrest any movement)

(The requirements of RCDs for fault protection in TT systems is in 531.3.5.3 and particularly relevant is the gloriously numbered 531.3.5.3.2.201 which talks about RCD protected outgoing circuits in a metal consumer unit and the supply-side incoming tails. It doesn't spell out the answer though)

Hope that helps. In this instance section 2.2.6 of the on site guide is a lot simpler to understand than the underlying regs (and I don't say that lightly!)

 

[davesparks]

A time delayed rcd is not required.

Why not? We have no information about what protection is required for any of the circuits so how can you make that decision?

 

[mainline]

Why not? We have no information about what protection is required for any of the circuits so how can you make that decision?

Because a time delayed RCD is not a requirement as long as certain conditions are met.

 

[HappyHippyDad]

Because a time delayed RCD is not a requirement as long as certain conditions are met.

Hi Mainline.
Could you outline those conditions for the original poster please?

 

[mainline]

Hi Mainline.
Could you outline those conditions for the original poster please?

This should explain it better than i can.

RCDs, metal consumer units and TT installations | NAPIT

NAPIT's Bill Allan answers another common contractor query. Contractors sometimes ask whether it's necessary to install an RCD in a TT installation before a metal consumer unit (i.e. a 'front end' RCD

professional-electrician.com

Basically ensure the tails are clipped and glanded properly and use rcbos.

The use of a split load metal consumer unit is not really suitable on a TT.

 

[davesparks]

Because a time delayed RCD is not a requirement as long as certain conditions are met.

But you stated categorically that it is not required with no mention of any conditions to this.

 

[mainline]

But you stated categorically that it is not required with no mention of any conditions to this.

A TIME DELAYED rcd is not required, the op said he was using rcbos.

Required means: compulsory, or otherwise considered essential; indispensable.

 

[davesparks]

A TIME DELAYED rcd is not required, the op said he was using rcbos.

Required means: compulsory, or otherwise considered essential; indispensable.

The OP asked if he can use RCBO's


Again, how do you know it is not required with no details of the installation? We have no details of the installation at all, other than it has a TT supply and may have RCBO's, MCB's or a mixture of both.

Without further detail I fail to see how you can state that a time delayed RCD is not required.

Even if this is a simple domestic installation there may be circumstances where a time delayed RCD is required.

 

[mainline]

The OP asked if he can use RCBO's


Again, how do you know it is not required with no details of the installation? We have no details of the installation at all, other than it has a TT supply and may have RCBO's, MCB's or a mixture of both.

Without further detail I fail to see how you can state that a time delayed RCD is not required.

Even if this is a simple domestic installation there may be circumstances where a time delayed RCD is required.

I think you are missing the point.

It doesn't make any difference not knowing about the installation, it doesn't change the fact that:

The use of a timed delayed RCD is NOT a requirement.

 

[pc1966]

I thought TT systems needed to have DP isolation if tripped (would have to look that up though) and not all RCBOs offer that?

So for some cases you might not need the up-front delay RCD, but it is not a simple 'no' due to that aspect. Also being a bit paranoid I would rather not have a failed/sticky RCBO allowing the whole installation CPC (and any connected external metalwork) to be elevated to near full mains voltage, with an up-front delay RCD you at least avoid uch a single point of failure.

 

[westward10]

Isolation on TT should isolate all poles, circuit protection only needs to disconnect the line conductor/s.

 

[mainline]

I thought TT systems needed to have DP isolation if tripped (would have to look that up though) and not all RCBOs offer that?

So for some cases you might not need the up-front delay RCD, but it is not a simple 'no' due to that aspect. Also being a bit paranoid I would rather not have a failed/sticky RCBO allowing the whole installation CPC (and any connected external metalwork) to be elevated to near full mains voltage, with an up-front delay RCD you at least avoid uch a single point of failure.

Although SP rcbos comply, I do agree that DP rcbos are the way to go whether TN or TT not just from the safety aspect but also the fault and testing side of it.

 

[brianmoooore]

.......................................stupid forum rule!

 

[brianmoooore]

Isolation on TT should isolate all poles, circuit protection only needs to disconnect the line conductor/s.

N-E fault would trip the RCD if RCBOs didn't break both poles. this would contravene the one out, all out reg.

 

[davesparks]

I think you are missing the point.

It doesn't make any difference not knowing about the installation, it doesn't change the fact that:

The use of a timed delayed RCD is NOT a requirement.

Maybe I amissing your point, please do explain what your point is??

 

[mainline]

Maybe I amissing your point, please do explain what your point is??

Ade1309​

“ I understand that I NEED a 100 amp time delayed RCD main switch”


The point is that you don't “NEED” a time delayed RCD it's one of the options, it's NOT a requirement.

 

[mainline]

N-E fault would trip the RCD if RCBOs didn't break both poles. this would contravene the one out, all out reg.

Are you saying that every consumer unit that's fitted with SP rcbos aren't compliant ?

 

[timhoward]

I think the intention of both myself and @mainline (we posted at same second)
was to encourage the OP to look into this more. Saying that an S-type up front RCD is needed implied that they hadn’t fully looked at the regs concerned or the OSG’s interpretation of them. I said “it’s not a given”but I’m sure we both meant the same thing.

I could have just answered the question but when someone’s first ever post on here is about installing a TT consumer unit I didn’t want to assume they should be doing the job in the first place.
I have to say that the regs are far and scattered concerning this and it’s little wonder that someone new would find it hard to understand them.
I normally don’t rate the OSG but on this topic it’s very helpful.