25mm 120 amp 3phase question

[Jim2287]

If i have...

3 phases
70mm cables
120 Amps
BS88s

....feeding a busbar chamber
and then coming off the busbar in 25mm, single phase, to feed a switch fuse of 60/80/100 amp rating ... all run in steel trunking, all singles cables, all cable entries protect cables with paxelin(spelling?)


the 25mm is protected against overload by the 60/80/100 SwFuse
but 25mm can only take ~101amps single phase, ~89amps 3 phase if a fault occured on the cable between SwFuse and Busbar


Would this be ok as the fault is very unlikely to happen amd a fault would blow the 120amp BS88 in very little time (so long as earth loops are good enough)... or is this not allowed ?

I would use the single phase current carrying capacity value, as only one phase feeds each of the SwFuses, right ?


any info appreciated

 

[ricicle]

You need the Ze at the busbar chamber so that by using the adiabatic equation you can work out if the 25mm² is protected by the 120A fuses in the event of a short circuit.The fuses in the S/F will (if sized correctly) provide the overcurrent protection.
I have covered this in someones previous post but don't feel like looking back at the moment as its Friday. If you can get back with the Ze then that will help.

 

[Jim2287]

Cant get it atm, so says its maximum for TN-S of 0.8ohms, would 25mm be allowed then or ?

what is the adiabatic equation for cable sizing?



thx

 

[crystal ball]

AFAIK you can use the tails of the same size as the outgoing fuses will allow for short lengths. If say you have a 400amp busbar and you want to tap off to a 32amp fused switch and had to use tails rated for 400amp they would not go in the terminals

 

[Jim2287]

hmm true ... i suppose its sort of the same thing as putting a spur off a ring final circuit .... 32amp breaker for ring, but a single 2.5 for the spur cant take 32 amps, so isnt protected against a fault current unless the current blows the 32amp breaker, same as the cable feeding the S/F isnt protected unless the fault blows the 120 amp fuse

 

[Spark123]

The adiabatic equation is S=sqrt(I²t)/K

K value for 70º general purpose pvc cable 70deg C copper cable is 115.
K value for steel conduit with an assumed initial temp of 50ºC and final temp of 160ºC is 47.
You will need to determine the value of fault current which will flow in the circuit under both earth fault and short circuit conditions, from this you should can look up the time it takes for the 120A fuse to disconnect for both these values and apply the above values to the adiabatic equation.

 

[Jim2287]

Know of better current time graphs than in the regs ? not too easy to read them very accurately


K cable 115
K steel 47
do i add them together for earth fault or use the lowest of the two ?

 

[Spark123]

You need to find out the pscc, pefc and the time it takes for the device to operate. The way I understand it is you need to apply it to the adiabatic for each of the paths, i.e. pscc will be through phase conductors so the size of the conductor needs to exceed the adiabatic using pscc and K=115. For pefc the size of the phase conductor needs to satisfy the adiabatic using K=115, also csa of steel conduit needs to exceed the adiabatic using K=47. You also need to take into accout parallel paths for earthing.
A second opinion of the above will be nice btw.

 

[Jim2287]

PEFC is Voltage / Earth loop ... right ? or is it just the restance of the earth path alone ?

and PSCC would just be phase to neutral short ? ... so at the incoming mains, with no Main bonds in place, the PEFC would be higher than PSCC, so if the PEFC blows the fuse, the PSCC will too

?

sorry for all the questions, my college teachers arent the best

 

[Spark123]

PEFC = prospective earth fault current - max current which can flow to earth via any of the phases.
PSCC = prospective short circuit current, phase-phase if your tester can handle 415v or double the max current which can flow between phase-neutral.
Most EFLI testers have a PSCC test built into them nowadays which can be used for both of the formentioned tests.

 

[Jim2287]

Ok thanks for the definitions

Our tester has PSCC on the earth loop

If im feeding switchfuse with just one phase, i wouldnt need to do PSCC phase to phase would i ? i would just do the phase-neutral as the fuse is single phase

or would i need to do phase-phase because there is a possibilty the two could short in the busbar ?

 

[Spark123]

If it is single phase then the PSCC would be phase to neutral.
I got it into my head you were talking 3phase for some daft reason

 

[Jim2287]

the busbar is 3 phase but each phase will feed a different Switch fuse (single phase switch fuses)

would this mean i use phase to phase PSCC ? because it is possible for a phase to touch to another phase if insulation broke down enough and they were touching


is it not best to use just 230volts (phase-neutral) because that will give you the lower Over-current, so gives you the longer disconnection time (sort of like worst case scenario)
if i use 415 volts the overcurrent would be high, so the fuse will blow fast, but if i use 230volts then there would still be overcurrent running in the cable, but for a longer time ( if u get what i mean)

 

[ricicle]

You would need to do phase to phase as you rightly said as there is the possibility of a short in the busbar chamber.
With your theoretical Ze of 0.8:-

t = k²S²/I² = 115²25²/600² = 8265625/360000 = 23 secs

A 600 amp fault would clear a 125 amp fuse (125A is a standard BS88 rating) in 10 seconds so no damage will occur to a 25mm² 70°c cable.
The smaller feed must not exceed 3m in length and be installed to minimise the risk of a fault and fire and danger to persons

 

[Jim2287]

thanks,

t = k²S²/I²

thats in the regs ?