AJJewsbury:

Disconnection times (0.4 and 5s) can not be met or achieved via the thermal element in UK/EU MCBs and hence rely on the magnetic (solenoid) trip function. This means that even the max listed EFLI values for type B-D 125 amp MCBs will result in disconnection faster than 0.1 seconds. This leaves us with fuses/devices 125 amps and over which typically have a Zs of <0.25 ohms. Going by ohms law 0.2 gives us 265kw or 1,150 amps with an infinite source.

.Those sound like rather odd assumptions to me. Many existing UK installations still use fuses for final circuits (either to BS 3036 or what was BS 1361) - anything upwards of 5A - not to mention the ubiquitous 13A fuse in FCUs. While fuses for small final circuits are certainly less fashionable for new work at the moment, they're still permitted so their absence can hardly be a safe assumption for basing disconnection time calculations on. Perhaps more to the point fuses were just about the only option (certainly domestically) when the 5s disconnection time was first introduced - so can't have been the original thinking. Even when MCBs are used for final circuits it's again common practice to use fuses far smaller than 125A for distribution circuits - I have a couple of 63A HBC fuses on distribution circuits at home and even the DNO's fuses are usually between 60A and 100A in the UK so even in that case plausible fault currents are in the region of a few hundred amps rather than thousands.  BTW - it's not quite true that the thermal element of MCBs can't cause the device to open within 5s - it's just that for most cases the current required to do that will trigger the magnetic part first. For some D-type MCBs 5s operation can certainly be achieved using just the thermal element using a lower fault current than needed for magnetic operation (see table 41.3 of BS 7671 for example).


Also keep in mind that most UK public supplies are tapped to deliver around 250 or 253V at source (to counteract voltage drop in the LV distribution network) - so even with a moderate fault current it's likely there'll still be close to the 230V at the source.


  - Andy.

True, however look at the time current curves of minature circuit breakers. 0.4 seconds often can not be achieved with typical PFC, so a solenoid coil is added to the breaker. Am I correct to say that even at maximum allowable EFLIs, breakers 100 amps and under will trip in their magnetic region? 


I am willing to acknowledge that way back when it was assumed the body could tolerate higher touch voltages for much longer periods of time. 


A 60 amp circuit still going to tax the source.

AJJewsbury:

Disconnection times (0.4 and 5s) can not be met or achieved via the thermal element in UK/EU MCBs and hence rely on the magnetic (solenoid) trip function. This means that even the max listed EFLI values for type B-D 125 amp MCBs will result in disconnection faster than 0.1 seconds. This leaves us with fuses/devices 125 amps and over which typically have a Zs of <0.25 ohms. Going by ohms law 0.2 gives us 265kw or 1,150 amps with an infinite source.

.Those sound like rather odd assumptions to me. Many existing UK installations still use fuses for final circuits (either to BS 3036 or what was BS 1361) - anything upwards of 5A - not to mention the ubiquitous 13A fuse in FCUs. While fuses for small final circuits are certainly less fashionable for new work at the moment, they're still permitted so their absence can hardly be a safe assumption for basing disconnection time calculations on. Perhaps more to the point fuses were just about the only option (certainly domestically) when the 5s disconnection time was first introduced - so can't have been the original thinking. Even when MCBs are used for final circuits it's again common practice to use fuses far smaller than 125A for distribution circuits - I have a couple of 63A HBC fuses on distribution circuits at home and even the DNO's fuses are usually between 60A and 100A in the UK so even in that case plausible fault currents are in the region of a few hundred amps rather than thousands.  BTW - it's not quite true that the thermal element of MCBs can't cause the device to open within 5s - it's just that for most cases the current required to do that will trigger the magnetic part first. For some D-type MCBs 5s operation can certainly be achieved using just the thermal element using a lower fault current than needed for magnetic operation (see table 41.3 of BS 7671 for example).


Also keep in mind that most UK public supplies are tapped to deliver around 250 or 253V at source (to counteract voltage drop in the LV distribution network) - so even with a moderate fault current it's likely there'll still be close to the 230V at the source.


  - Andy.

True, however look at the time current curves of minature circuit breakers. 0.4 seconds often can not be achieved with typical PFC, so a solenoid coil is added to the breaker. Am I correct to say that even at maximum allowable EFLIs, breakers 100 amps and under will trip in their magnetic region? 


I am willing to acknowledge that way back when it was assumed the body could tolerate higher touch voltages for much longer periods of time. 


A 60 amp circuit still going to tax the source.