cable behind 50mm insulated plasterboard

[davesparks]

OK, but why?

I'm struggling to put the answer in to written words that will make sense at the moment, but as I understand it, it's basically along the lines of this.
The neutral and the installation's earth are not directly connected to each other so can be at different potentials in a TT supply under normal conditions, in a TN supply they are directly connected together and to the same physical earth connection.

 

[nicademus]

Thanks Dave for the reply, your thread seems to go along with what my understanding was. Many thanks.

 

[SimpleSimon]

I'm struggling to put the answer in to written words that will make sense at the moment, but as I understand it, it's basically along the lines of this.
The neutral and the installation's earth are not directly connected to each other so can be at different potentials in a TT supply under normal conditions, in a TN supply they are directly connected together and to the same physical earth connection.

Think that is a pretty easy to follow explanation.
Imagine there is a N-E fault on your lighting circuit on a TT system. If you have on the lighting circuit only SP RCBO this is going to go undetected. But further upstream you have a double pole RCD main switch which is going to trip. Because of this scenario, you can't re-energise your system. If you had SPSN RCBOs installed then this would trip first narrowing down your fault to a lighting circuit (remembering that our main switch is 100mA)

 

[D Skelton]

Think that is a pretty easy to follow explanation.
Imagine there is a N-E fault on your lighting circuit on a TT system. If you have on the lighting circuit only SP RCBO this is going to go undetected. But further upstream you have a double pole RCD main switch which is going to trip. Because of this scenario, you can't re-energise your system. If you had SPSN RCBOs installed then this would trip first narrowing down your fault to a lighting circuit (remembering that our main switch is 100mA)

A SP RCBO will still trip, it just won't disconnect the fault, thus leaving it to the downstream DP RCD to perform that task.

 

[SimpleSimon]

Course it will!!

Sorry, I've edited my post now, cheers for the correction

 

[Engineer54]

A SP RCBO will still trip, it just won't disconnect the fault, thus leaving it to the downstream DP RCD to perform that task.

Think you meant to say ''Upstream'' here!! lol!!

 

[D Skelton]

Think you meant to say ''Upstream'' here!! lol!!

Upstream/downstream, who cares lol

Everyone knows I mean the one BEFORE it

 

[GEOFF S]

Imagine two propertys, both TT, one of the houses has no rcd protection and has developed a Line to earth fault. Both propertys have a common lead water pipe.

If i was working in the other house what risk would there be?

Cheers

 

[trev]

If it had no RCD would there not be another type of protection?
What about what the DNO put in their cut out thingys, what are they called?

 

[GEOFF S]

If it had no RCD would there not be another type of protection?
What about what the DNO put in their cut out thingys, what are they called?

Unfortunately on a TT the dno fuse is unlike to operate due to the high impedance earth fault path.

Cheers

 

[Silly Sausage]

If it had no RCD would there not be another type of protection?
What about what the DNO put in their cut out thingys, what are they called?

No idea, I've never seen inside one!

 

[trev]

Unfortunately on a TT the dno fuse is unlike to operate due to the high impedance.

Cheers

So the trick is to make sure the impedance is low enough that any fuse will operate.
It's not difficult, I do it all the time. My own home is on a TT and has a sub 1 ohm impedance. If memory serves me well it's around the 0.5 - 0.6 mark (can't be bothered to go and get the cert) my record is 0.29.

 

[GEOFF S]

So the trick is to make sure the impedance is low enough that any fuse will operate.
It's not difficult, I do it all the time. My own home is on a TT and has a sub 1 ohm impedance. If memory serves me well it's around the 0.5 - 0.6 mark (can't be bothered to go and get the cert) my record is 0.29.

Impressive, are we talking Ra or Zs?

Hot or cold site?

Cheers

 

[trev]

Well if we want to be pedantic it's actually a Ze although the terms are interchanged all the time when talking about TTs. Ra is the resistance of the electrode itself measured with the spike system. The 0.29 was a domestic

 

[GEOFF S]

Well if we want to be pedantic it's actually a Ze although the terms are interchanged all the time when talking about TTs. Ra is the resistance of the electrode itself measured with the spike system. The 0.29 was a domestic

Well to be pedantic, where RA isnt Known it may be replaced by Zs, 411.5.3. So the RA was 0.29 ohms?