Dealing with TT under the 17th.

securespark · 1 Jul 2008

Anyone given any thought to how to fit a CU to comply with the 17th to a TT-fed installn?

nozspark · 1 Jul 2008

Same as any other 17th CU install - High integrity board with rcbo's for and circuits on the isolator side!!

RF Lighting · 1 Jul 2008

The best soloution for any supply type is a board full of RCBOs

nozspark · 1 Jul 2008

Can you get people to pay for a CU full of RCBO's

RF Lighting · 1 Jul 2008

Thats the price. Take it or leave it.

I don't do domestic, so can't really comment, but I don't think you will be seeing many dual split RCD boards in commercial / industrial installs

securespark · 1 Jul 2008

I did a quote for a 17th board (8 way with 100A incomer, plus 8 RCBO's (6 utilised) and MEB's.

Let's just say I think the customer was a tad surprised....

Taylortwocities · 1 Jul 2008

Think about it people.

For most TT installs a dual 30Ma RCD CU will comply.

If you have a special circuit (like a fire alarm on a standard MCB with cable less than 50mm) then you may need to have a 100ma type S RCD incomer: but generally the dual 30RCD will do it for you.

Spark123 · 1 Jul 2008

If it is a normal cable concealled less than <50mm then a 30mA will need to be installed.
Another point where the regs are a bit of a mess, type S RCDs (BSEN) need to disconnect within 300ms, a majority of circuits in a TT installation need to disconnect within 200ms.

securespark · 1 Jul 2008

That was what I was bothered about. But I suppose you can just use 30mA RCBO's and hang the expense!

Or convert to PME...

Spark123 · 1 Jul 2008

I'm glad they have admitted the 2x test was a printing error though

My preferred method for today is the old split load with RCBOs on the non RCD side.
It is just me that doesn't like the idea of having lighting on the same RCD as sockets?

Click-Sure · 1 Jul 2008

A PME conversion in my area (central networks) is £80.

I just weigh things up against that.

Lectrician · 2 Jul 2008

PME in WPD area is £18.

NIC Connections mag states TT with a seventeeth board is fine - 100mA time delay not required.

Lectrician · 2 Jul 2008

However, having an RCBO on a single circuit o a TT......I am unsure and need to read and investigate, but I think it may have to be a double pole RCBO and not a single pole type. I always recall that on a TT, the neutral must also be isolated by the device.

Ponders........

dingbat · 2 Jul 2008

Spark123 said:
I'm glad they have admitted the 2x test was a printing error though

It wasn't. (I'm assuming you mean the amendment to Note 2 on page 46?)

In fact it was a perfectly valid statement, not referring to any test at all but based on expected minimum values of earth fault current, given maximum acceptable electrode resistance, Ze and a 500mA max RCD.

oharaf · 2 Jul 2008

Lectrician I have the same feeling as you that on TT the neutral must also be isolated, just can't think where I came across this, fairly recent thou.

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Dealing with TT under the 17th.

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