40A shower disconnection time

[ipf]

So why does anything need to disconnect in 0.4s then? Life saving RCDs can trip up to 0.3s, thats only 1/10 slower.

That's 300 milliseconds not 30.

 

[oscar21]

That's 300 milliseconds not 30.

The max disconnection time of a 30ma RCD is 300ms, which is 0.3 seconds, at x5 or 150ma the max disconnect is 40ms which is 0.04 seconds.

 

[ipf]

The max disconnection time of a 30ma RCD is 300ms, which is 0.3 seconds, at x5 or 150ma the max disconnect is 40ms which is 0.04 seconds.

oops! oscar'd. need me glasses on.

 

[pc1966]

The max disconnection time of a 30ma RCD is 300ms, which is 0.3 seconds, at x5 or 150ma the max disconnect is 40ms which is 0.04 seconds.

Those trip current / time values are intended to keep you in the AC-1 to AC-2 areas, so little risk of fibrillation, of the shock risk model:

Electrical injury - Wikipedia

en.wikipedia.org

The 0.4s standard for TN earthing, and the related 0.2s for TT earthing, follow similar arguments about keeping touch voltage (about U/2 for TN fault as R2 roughly like R1, more or less U for TT) and exposure time during the fault such that typical body resistance won't result in deadly current-time exposure. But that only sort of works for typical dry contact, hence in a bathroom the old (pre-RCD everything) standard for supplementary bonding to further reduce the local voltage differences during a fault.

 

[davesparks]

So why does anything need to disconnect in 0.4s then? Life saving RCDs can trip up to 0.3s, thats only 1/10 slower.

Maybe 1/10 slower but at a vastly different fault current!
The RCD trips at 30mA in 0.3s, that fast enough to save most people from a fatal shock.
The 40A MCB trips at a current in the hundreds of amps in 0.4s

The 0.4s is to keep touch voltages down, prevent damage to cables from long durations of high current faults and that sort of thing.

 

[oscar21]

Maybe 1/10 slower but at a vastly different fault current!
The RCD trips at 30mA in 0.3s, that fast enough to save most people from a fatal shock.
The 40A MCB trips at a current in the hundreds of amps in 0.4s

The 0.4s is to keep touch voltages down, prevent damage to cables from long durations of high current faults and that sort of thing.

That cant be right, the RCD "starts" to trip at 30mA but it depend what bit of the sinewave its at, in 300ms it could be peak voltage and current.

If you've ever had a shock from an RCD protected circuit you'll know it hurts just as much as non RCD, it just doesnt last as long.