Refurb works In offices

[Ian1981]

I wasn't aware that there is a required disconnection time for short circuit.

Needs to disconnect in 0.4 of a second does it not for TN systems.
If your disconnection times are met for earth faults then it’s not a requirement to calculate or measure short circuit as the likelihood is this will be satisfied if your zs times are met, however if you cannot meet your earth fault loop impedance times though a protective device like an mcb and rely on the rcd for the earth faults only then short circuit times still need to be met don’t they?
No good having a live to neutral fault and taking 10 seconds for example to disconnect is it?

 

[Deleted member 9648]

Where in Bs7671 does it give disconnection times for short circuit faults?
Happy to be corrected on this but I'm not aware of any disconnection time requirement on short circuit faults, only on faults to conductive parts or protective conductors.

 

[Ian1981]

It doesn’t but do you not see what I’m saying?
If your zs is so high that you need to rely on a rcd to trip on an earth fault we’ll whos to say your live to neutral short circuit will trip before causing damage to the cable insulation or fire risk or giving someone an electric shock as the rcd will not operate on a short circuit.

When your zs times are satisfied then it’s assumed that short circuit times will be met especially with twin and earth as the cpc has a smaller CSA.

This is also referenced in GN8 when using an rcd to meet disconnection times for earth faults
Another is to provide localised supplementary equipotential bonding

 

[Deleted member 9648]

But a high Zs may be the result of a high Ze which will have no effect whatsoever on short circuit current. In a properly designed circuit the live conductors will necessarily be of low impedance or volt drop in normal use will be excessive, therefore short circuit current will be of sufficient magnitude to operate an overcurrent device chosen to coordinate with the circuit conductors current rating. And I cant see what supp bonding has to do with short circuit current.

 

[Ian1981]

When did I ever say supplementary bonding was for short circuit faults?
Earth faults if you read it.
The ops circuits may not be designed properly which is why he might have a perticular high zs on the ring circuit.
His zdb was ok as he has stated so he may have a rather large circuit so my suggestion and it’s only a suggestion is to check the live neutral loop is also not too high as the mcb needs to operate on short circuits as well.
An rcd May take care of the earth fault but not a short circuit.

 

[Richard Burns]

Where in Bs7671 does it give disconnection times for short circuit faults?
Happy to be corrected on this but I'm not aware of any disconnection time requirement on short circuit faults, only on faults to conductive parts or protective conductors.

I think this would be referencing 434.5 where any fault current must be disconnected without damage to the conductors and the time to reach maximum temperature is calculated from t= (k²S²)/I².

 

[Deleted member 9648]

Thanks for that Richard.

 

[cliffed]

Thanks all,will once again tell those higher than me,this circuit needs a rcd.
With other additional power points in other areas,there’s been no problem with the loops,it’s just in this area.

 

[Ian1981]

I would say that given you’ve installed and moved socket outlets that at the very least your works will require rcd protection to the scope you have carried out (rcd protection to sockets, cables which you have installed in walls )

 

[cliffed]

Agree,but who’s going to pay this extra,I have no response from anyone yet on site.