2391 questions

[Steve D]

Hi Guys,

I've been doing a few past papers for 2391 and I think my brain is a bit mashed now. I'm struggling to get my head around a few of the questions, maybe you guys can assist.

1) State 3 wiring systems that would not normally require a separate thermoplastic (pvc) insulated conductor as a cpc.

2) When testing ring final circuit, state the reason why L-N readings at each socket would increase towards the midpoint of the ring and then decrease again.



Cheers
Steve

 

[markhtly]

Hi Guys,

I've been doing a few past papers for 2391 and I think my brain is a bit mashed now. I'm struggling to get my head around a few of the questions, maybe you guys can assist.

1) State 3 wiring systems that would not normally require a separate thermoplastic (pvc) insulated conductor as a cpc.

2) When testing ring final circuit, state the reason why L-N readings at each socket would increase towards the midpoint of the ring and then decrease again.



Cheers
Steve

1.Metal conduit,mineral insulated cables and ermm...

2.Incorrectly identified ring ends perhaps.

 

[Des 56]

1 An installation comprising cables with cpc in the sheath,twin/earth
An extra low voltage system
Double insulated conductors

2 Do you not mean L-E increases to the mid point and not as stated L-N

When you cross connect the conductors with the same size cores it will be substantially the same over the whole of the ring

The L-E cross connecting will give you the readings you quote, because of the reduced csa of the cpc such as in twin and earth cable

 

[Steve D]

mark is right. if the legs are incorrectly identified then the readings will get higher as you get to the middle of the ring.

even when you cross connect L-cpc, the readings shouldn't increase like this, they should end up being substantially the same as eachother

 

[Guest123]

Hi.

1 - Steel conduit, SWA, MICC

2. If the ring was broken at then mid point you would then in effect be measuring the resistance of two radials, increasing to the end of the first leg then decreasing as you go down the second leg.

Or a bridged ring.