2391 question

[i=p/u]

have 400/230v supply tn-c-s system with 35mm supply conductors..(ze 0.1 ohms) there is an extension built which is a 30m run of 3 core swa with 25mm conductors, one core is used as circuit protective conductor

Q , state giving reason wheather the copper cpc incorparated in distribution circuit is suitable as part of the main equpotential bonding to gas and water services installed to the new building from public supply

Q, descibe a suitaqble test to confirm the continuity of cpc in distribution cable

 

[i=p/u]

but if the 25mm condutor is going to MET how does this reach the extraneous conductive parts... probably thinking to hard now, thanks wise ones, later and thx

 

[ringer]

You mean assuming the conductor has enough capacity to do both jobs? To be honest - I can't think of a regulation that specifically forbids it. But I would have thought it would be bad practice. And as stated in an earlier post by yourself, circulating currents may generate enough heat to affect the other conductors in the cable. Is that not why bonding cables are usually run separately from all other circuits?

 

[R4BOE]

hi spin As for the circulating neutral currents, I don't think they care what the conductor is called, if it's there and it's connected, they'll use it.

i am only going with what is prescribed in gn 8

 

[R4BOE]

but if the 25mm condutor is going to MET how does this reach the extraneous conductive parts... probably thinking to hard now, thanks wise ones, later and thx

i think thats why there is a high fail rate on the 2391 cos some times the questions are poorly worded and people are unsure what they are looking for as an answer

 

[R4BOE]

You mean assuming the conductor has enough capacity to do both jobs? To be honest - I can't think of a regulation that specifically forbids it. But I would have thought it would be bad practice. And as stated in an earlier post by yourself, circulating currents may generate enough heat to affect the other conductors in the cable. Is that not why bonding cables are usually run separately from all other circuits?

if you look at gn 8 page 52 it shows a pme supply to a separate building using the cable as a combined cpc and bonding conductor as illustrated

also worth noting is when your maximum zs can not be meet you can use supplamentay bonding to achive this

 

[i=p/u]

15. An earth electrode resistance test has been carried out, and the following results were obtained: 128Ohms, 120Ohms and 134Ohms. Determine the value of RA to be recorded on the certificate. Show all calculations.

il get by with a little help, ty

id say add them together and divide by 3 to get awnser... i dont think it wants you to go into the effectiveness of the values given..help help i think

 

[i=p/u]

anyone

 

[Deleted member 26818]

The value of Ra is the mean or average of the of the three readings.
So yes, add the three readings together, and divide by three.

 

[i=p/u]

just there is more than 5% on some of them and we were taught about deviation and a longer sum... but im sure they just looking for the average ty

 

[i=p/u]

10 State the effect each of the following would have on the measured insulation resistance of a conductor.

a) decrease in length increase

b) decrease in CSA none

c) decrease in temperature none

 

[spark 68]

Except a) = increase

That almost looks like a mixture of two different 2391 questions

 

[Deleted member 26818]

Why do you believe that a decrease in conductor length will result in a decrease in insulation resistance?
What do you believe causes an increase or decrease in insulation resistance?

 

[i=p/u]

19 A 200mA general purpose RCD to BSEN 61008 is to be tested to ensure it operates within specification. State the

a) test instrument to be used rcd tester

b) the maximum test current to be applied during the test 500mA

c) the maximum acceptable tripping time during the test in b) above 40ms min 150 max

 

[i=p/u]

things like length of circuits , testing more than 1 ciruit at a time

Why do you believe that a decrease in conductor length will result in a decrease in insulation resistance?
What do you believe causes an increase or decrease in insulation resistance?

 

[spark 68]

RCD question

b) 200mA
c) 300ms

only 30mA (or less) RCDs used for additional protection are tested @ 5X I delta n

general purpose RCD to BSEN 61008 max = 300ms

general purpose others max = 200ms