Calculating Maximum permitted cable length with maximum Zs value?

[Jabbajaws]

How do l calculate the maximum cable length for a maximum Zs value?

Isn't there also correctional factors to be applied for ambient temperature?

 

[BillR3]

Hi Jabbajaws

There are so many parameters when trying to work out a cable length specific to Zs max value. You need to know the circuit load, associated derating factors to allow you to select an appropriate cable from Appendix 4 of BS 7671. Then you have to work out the voltage drop taking into consideration what type of load it is for, power or lighting ? The formula Vd= mV/A/m x Ib x L and divide by 1000 will give you the actual volt drop and the figures again can be selected from Appendix 4.
You then need to know the Ze value at the incoming supply point which will allow all your calculations to be verified. To work out the Zs value you then need to work out the R1 and R2 values by using the formula:
R1= r1 ( resistance per meter from table 9A from on site guide ) x circuit length x temp correction factor (from table 9C from on site guide)
The R2 value is worked out in exactly the same manner and once you have all these results then you use the formula ZS = Ze + R1 + R2 this will allow you to see if the circuit protective device Zs value is acceptable and which will allow the safe disconnection of the circuit in the event of a fault condition.
Earth fault current is worked out from If = Uo/Zs
From these values you can work out the minimum value of the CPC for the circuit by the following:

S min = square root of the following, If squared x t ( disconnection time ) then divide by the appropriate K factor from tables 54.2 to 54.6 from BS 7671

May sound a bit confusing but there are so many factors when it comes to cable lenghts that you really have to go into minute detail to come up with the safe and correct size.

Hope this helps,

BillR3

 

[boballum]

Buy Amtech software the cable calc is great really easy to use, also prints off a proffesional looking report for your customers

Hi Jabbajaws

There are so many parameters when trying to work out a cable length specific to Zs max value. You need to know the circuit load, associated derating factors to allow you to select an appropriate cable from Appendix 4 of BS 7671. Then you have to work out the voltage drop taking into consideration what type of load it is for, power or lighting ? The formula Vd= mV/A/m x Ib x L and divide by 1000 will give you the actual volt drop and the figures again can be selected from Appendix 4.
You then need to know the Ze value at the incoming supply point which will allow all your calculations to be verified. To work out the Zs value you then need to work out the R1 and R2 values by using the formula:
R1= r1 ( resistance per meter from table 9A from on site guide ) x circuit length x temp correction factor (from table 9C from on site guide)
The R2 value is worked out in exactly the same manner and once you have all these results then you use the formula ZS = Ze + R1 + R2 this will allow you to see if the circuit protective device Zs value is acceptable and which will allow the safe disconnection of the circuit in the event of a fault condition.
Earth fault current is worked out from If = Uo/Zs
From these values you can work out the minimum value of the CPC for the circuit by the following:

S min = square root of the following, If squared x t ( disconnection time ) then divide by the appropriate K factor from tables 54.2 to 54.6 from BS 7671

May sound a bit confusing but there are so many factors when it comes to cable lenghts that you really have to go into minute detail to come up with the safe and correct size.

Hope this helps,

BillR3

 

[Jabbajaws]

Cheers mate, sounds like you definitely know your stuff...

 

[ian.settle1]

Buy Amtech software the cable calc is great really easy to use, also prints off a proffesional looking report for your customers

Problem with Amtec and Hevacomp it can not use common sense when calculating cable size as if you used all the derating factors as you are suppossed to then you could end up using 16mm for a lighting circuit.

 

[platty1005]

If it is a single phase circuit then why not use the tables in the On site guide Section 7 page 43

If you know your Ze, Protective device type, cable size and installation method then the table gives you the maximum length

If you are not sure post the above information and I will look it up for you