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.

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.

Am I correct to say that even at maximum allowable EFLIs, breakers 100 amps and under will trip in their magnetic region?

A 60 amp circuit still going to tax the source.

AJJewsbury:

Am I correct to say that even at maximum allowable EFLIs, breakers 100 amps and under will trip in their magnetic region?

Not entirely - BS 7671 allows higher EFLIs for D types - e.g. 0.68Ω (table 41.3) for a D32 for 5s disconnection time - so minimum fault current of around 338A  - yet 20xIn (640A) is needed to ensure magnetic operation.

 

A 60 amp circuit still going to tax the source.

Not much I would have thought - Max Zs for a 63A BS 88-3 fuse is 0.68Ω for 5s disconnection time - so again minimum fault current of 300-odd amps. Given that most public supply transformers in urban areas are rated at 400A or more per phase, in unfavourable conditions it wouldn't even count as an overload.

  - Andy.

Do have some time current curves for D types? You're right, but I just want to compare curves.


How often do these values actually approach 0.68ohms?

mapj1:
Am I correct to say that even at maximum allowable EFLIs, breakers 100 amps and under will trip in their magnetic region? 


Mostly true, and certainly true to say you will get prompt clearance for faults from live to to neutral on circuits that meet the full load voltage drop requirements of Lighting 3% other uses 5% .?


(and that allows us to get an approx  figure for the live path of half the L-N loop resistance)


Faults from live to earth need to be considered separately and problems really arise when the earth impedance is significantly higher - for example for a TNS service that is only just meeting the local DNOs spec the earth loop impedance may be as high as 0.8 ohms on a supply with a 100A fuse - so the PSSC to earth is 230/0.8 ~ 285A, but the LN loop may be more like (10V/100A 10 milliohms) 2kA short circuit prospective

Now that may not fire a C type 32A breaker on an earth fault, but it probably will, and it will certainly fire the more common B32 that we use for most socket circuits, if the fault is at or near the consumer unit end.

However a shower on a B50Amp breaker might need as much as 250A to instant trip, and if the external earth resistance is the maximum 0.8 ohms permitted it may or may not do so if the fault is at the remote end of more than about 30m of T and  E cable.

Of course in the last few years that MCB  would have become  a 50A RCBO, but there are a lot  in use out there that pre-date that change. (so remove the bathroom bonding in an old house at your peril.. )

M.


In the world of big stuff you may introduce programmed delays of 1sec or 3 sec or 5 sec to give some discrimination with downstream protection, but in practice 5 secs is a long time to be standing in front of something that is growling and smoking because one phase is shorted.




 

What are the typical real world Ze values being measured in domestics?  

ProMbrooke:

mapj1:
Am I correct to say that even at maximum allowable EFLIs, breakers 100 amps and under will trip in their magnetic region? 


Mostly true, and certainly true to say you will get prompt clearance for faults from live to to neutral on circuits that meet the full load voltage drop requirements of Lighting 3% other uses 5% .?


(and that allows us to get an approx  figure for the live path of half the L-N loop resistance)


Faults from live to earth need to be considered separately and problems really arise when the earth impedance is significantly higher - for example for a TNS service that is only just meeting the local DNOs spec the earth loop impedance may be as high as 0.8 ohms on a supply with a 100A fuse - so the PSSC to earth is 230/0.8 ~ 285A, but the LN loop may be more like (10V/100A 10 milliohms) 2kA short circuit prospective

Now that may not fire a C type 32A breaker on an earth fault, but it probably will, and it will certainly fire the more common B32 that we use for most socket circuits, if the fault is at or near the consumer unit end.

However a shower on a B50Amp breaker might need as much as 250A to instant trip, and if the external earth resistance is the maximum 0.8 ohms permitted it may or may not do so if the fault is at the remote end of more than about 30m of T and  E cable.

Of course in the last few years that MCB  would have become  a 50A RCBO, but there are a lot  in use out there that pre-date that change. (so remove the bathroom bonding in an old house at your peril.. )

M.


In the world of big stuff you may introduce programmed delays of 1sec or 3 sec or 5 sec to give some discrimination with downstream protection, but in practice 5 secs is a long time to be standing in front of something that is growling and smoking because one phase is shorted.




 

What are the typical real world Ze values being measured in domestics?  

Ze in my part of the woods (Norfolk U.K.) for TN-C-S average 0.20/0.30 Ohms.


For TT with an earth rod average 200 Ohms in damp well draining sandy soil.


Z.

Do have some time current curves for D types? You're right, but I just want to compare curves.

How often do these values actually approach 0.68ohms?