T T consumer unit upgrade

[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.