Electrical installation design guide

[mickfred]

Have been sizeing up the size of armour on diffent SWA cables to make sure they are suitable to be protective conductors. Come across table 8.8 page 100 .on this table it says maximum bs88 fuse size (for example) 120mm 4core is 500a, when it also says current carrying capacity is 335 method c and 353 method e.
Is this just one of the many mistakes in these books or I missing something

 

[1capybara]

Have been sizeing up the size of armour on diffent SWA cables to make sure they are suitable to be protective conductors. Come across table 8.8 page 100 .on this table it says maximum bs88 fuse size (for example) 120mm 4core is 500a, when it also says current carrying capacity is 335 method c and 353 method e.
Is this just one of the many mistakes in these books or I missing something

Because motors start with a spike in current, you want slow blow fuses much larger than the FLA of the motor or the conductor. American code allows you to size the fuse or circuit breaker at 2 or 3 times the size you would use based on the FLA of the motor.
As for using the armour of SWA as a CPC, this is allowed for all 3 phase circuits.

 

[mickfred]

I get that motors pull much more at startup (a few seconds at most ) but this table suggests that a 500amp fuses can protect a cable rated at 335 amps. As for armour being able to be a protective conductor on all size cables this is not true .

 

[Bobster]

Any motor circuit over 0.37kW must include thermal protection against overload. So where as the fuse is say 500A the thermal protection will protect the cable and the motor from currents higher than they are rated.

 

[1capybara]

About the SWA armour being used as a CPC in 3-phase circuits see this http://www.wiley.com/legacy/wileychi/eca_wiringregulations/supp/Appendix_16.pdf
Agreed? ....or maybe I do'nt understand?

 

[mickfred]

So in a factory an main cable feeding a bussbar system feeding lots of isolators feeding lots of different loads and motors will not have thermal protection ,so sticking a oversized fuse in the main isolator may not be a good idea

 

[mickfred]

I don't have it on me , but am sure in the iet earthing book it list diffent size SWA and some of them don't have the correct CSA to be used as a protective conductor .

 

[Pete999]

I don't have it on me , but am sure in the iet earthing book it list diffent size SWA and some of them don't have the correct CSA to be used as a protective conductor .

I don't have it on me , but am sure in the iet earthing book it list diffent size SWA and some of them don't have the correct CSA to be used as a protective conductor .

Did you get the downloads?

 

[mickfred]

Can't open it pete . what it say am guessing am wrong

 

[Pete999]

Can't open it pete . what it say am guessing am wrong

Sorry I did it 3 times wasn't sure I had got it, it is a pdf of SWA cable szes in conjunction with the size of the armoring and it's suitability size wise to be able to be used as a cpc, I 'll have another look. PM me your email and I'll try and send it that way

 

[mickfred]

Don't. Know how to pm . my email address is [email protected] ta

 

[mickfred]

Had a look online and basically any cable over 70mm doesn't have the CSA of armour to be used as a CPC, might be looking at a obsolete table

 

[1capybara]

Any motor circuit over 0.37kW must include thermal protection against overload. So where as the fuse is say 500A the thermal protection will protect the cable and the motor from currents higher than they are rated.

Rob, when we continuing reading Article 552.1.2 it says "this requirement does not apply to a motor incorporated in an item of current using equipment complying as a hole with an appropriate BS." which means if you have a motor which is part of a bandsaw or other machine 552.1.2 does not apply. right?

 

[mickfred]

Well apart from 120mm4 core.which is lucky cos that is what am putting in

 

[Wilko]

About the SWA armour being used as a CPC in 3-phase circuits see this http://www.wiley.com/legacy/wileychi/eca_wiringregulations/supp/Appendix_16.pdf
Agreed? ....or maybe I do'nt understand?

This is interesting new information (for me). I will have to read carefully and absorb etc. Up till now I've used the guide Pete has kindly published as I'm pretty sure it's safe.