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