disconnection times

[IQ Electrical]

Sorry IQ i meant to say just the socket outlets in the home or if fed from a garage say

Yes but they would likely be on a 'A1 Ring Final Circuit with 32A protective device so would have a TN system disconnection time of 0.4 seconds.

 

[snowplough]

Hi IQ,

Am in right in thinking then ,that for a L-E fault in the above scenario with the lawn mower for instance , the 0.4 s disc time is there to prevent elec shock and not cables being damaged causing fire etc. If this is the case then why in the bs7671 on page 46 ,note 1 at bottom of page says ,disconnection is not req for protection against elec shock but may be required
for other reasons such as thermal effects prevention?.

This is whats confusing me i think

kind Regards

LOCKET

 

[IQ Electrical]

Basic protection is the usual form of 'basic' shock protection ie. insulation of live parts etc.

Protection under fault conditions ensures that accessible conductive parts ie. metallic enclosures, pipes etc. must not be hazardous under single fault conditions.


ADS is used to limit the magnitude and duration of fault voltage between exposed, conductive and extraneous-conductive parts.


Where shock risk is increased (special locations etc.) then additional protection is required ie. RCD protection or supplementary bonding.
This also guards against failure of any of the basic/fault protection measures.

 

[Chr!s]

The 0.4 sec is part of ADS and is based on Biegelmeier and Lee's testing of human thresholds at 230 v.

Not sure about the 5 sec, but the bigger the circuit the larger the fault current required to disconnect, on large installations it will be difficult to generate enough current to disconnect within 0.4, so allowing upto 5 secs give some leeway.

 

[Chr!s]

Hi IQ,

Am in right in thinking then ,that for a L-E fault in the above scenario with the lawn mower for instance , the 0.4 s disc time is there to prevent elec shock and not cables being damaged causing fire etc. If this is the case then why in the bs7671 on page 46 ,note 1 at bottom of page says ,disconnection is not req for protection against elec shock but may be required
for other reasons such as thermal effects prevention?.

This is whats confusing me i think

kind Regards

LOCKET

Cables need to be protected against thermal effects using the adiabatic, now for short circuit time needs to be limited to ensure limiting temp of cable is not exceeded.

Now for a earth fault current there are two requirements, first we have ADS so disconnection times need to be met, secondly we again need to ensure that the adiabatic equation is met. Alternatively you may use table 54.7(but note second paragraph 543.1.1)