Disconnection Time Requirments?

[Mbrooke]

Why do circuits 32 amps and under require a 0.4 second disconnection time at 230 volts, but circuits over 32 amps are legally allowed 5 seconds? 115 volts for 5 seconds is enough to cause injury to the body.

 

[7anoter4]

In which publication did you find these requirements?

 

[Mbrooke]

BS7671

https://imgur.com/a/iN5TTvr#sIqT5t7

https://imgur.com/a/04PsLYW#rSfikU4

 

[7anoter4]

You are right. I found it my self in 2011 edition. I think it's a mistake. It has to be 0.5 instead of 5.
I think it is based on IEC 60479-1 eventually.
If we compare IEC 60479-1 Fig.20 for 50/60 Hz with Fig.22 for d.c. for 40 mA body current the time limit is 0.2 s for a.c. and 0.5 sec for 45 mA d.c.

 

[Mbrooke]

I'm still getting 5 seconds in IEC 60364-4-41:

You're right though, 5 seconds violates the body graph.

 

[Parchie]

For Condition 1 (defined as normally dry situations, where the surface on which the person at risk is standing presents some resistance (to the general mass of earth) and that a person is assumed to have dry or moist skin), the disconnection time could be 5 seconds or greater.
7anoter4 is correct in saying 0.5 seconds for Condition 2 (wet locations) though!

 

[bacon4life]

I had assumed the limits for small circuits were aimed at protecting end users versus for larger circuit sizes perhaps the requirements aimed more at fire prevention and/or equipment damage prevention while also allowing some coordination with branch circuit protection. The 32 A circuits seem much more likely to have plug/cord connected devices.

 

[Mbrooke]

Could the source impedance and voltage drop have something to do with it? A 32 amp circuit would make the source look strong, and as such the source can be considered infinite- providing all the current needed during a fault without voltage drop. However a fault on a 400 amp circuit would make the source look weak- dropping in voltage as it supplies the current?

@Bacon- but in theory the hazard is still there even for larger circuits and in some cases feeders too.