Fuses and MCB and BS7671:2001.

[kai]

According to Tables in the Brown copy of BS7671:2001, a 13amp BS1362 cartridge fuse has a max allowable earth resistance of 2.53.ohms for 0.4.sec disconnection...BUT a B16 amp MCB has a max earth loop resistance of 3.00.ohms for 0.4.sec disconnection.


It seems that the lower rated device needs a SMALLER earth loop resistance to comply with 0.4.sec disconnection...normally you will expect the larger device to require a smaller resistance to operate in 0.4.sec?

Is this right?, or am I interpreting the regulations correctly?

 

[plugwash]

they are different types of device with different requirements to trip them.

mcbs have a fast trip current. Below this point they will trip far too slowly to meet even 5s disconnect. Above this current they trip extremely fast (fast enough to meet all disconnect times requires by BS7671)

fuses on the other hand have a smooth curve with no distinct fast trip point.

 

[mapj1]

A 13A BS1362 plug top fuse doesn't actually blow very fast at all for small overloads...
Bussmann, for example, specify all their plug-top cartridge (BS1362) fuses (the TDC180 range ) to dissipate 1 watt or less at full load, irrespective of rating, and at 25 degrees to hold for a minimum of 30 minutes at 1.6 times rated current, and to blow within 30 minutes at 1.9 times rated current. Presumaby this range avoids premature failure in warmer environments, but still guarantees an acceptable minimum level of protection.

Datasheet at Farnell http://www.farnell.com/datasheets/55950.pdf
I guess at an overload of 1.6 times current, i.e. 20A or so, sockets and flex cables might heat up quite noticably in the (at least ) 30 minutes before things go ping ! Looking at the time/current curves, I'd expect with a high overload or a dead short, a 16A, (and probably for that matter a 32A) breaker to fire first, so actually the usefulness of the plug-fuse is really rather restricted, and I'd suggest that certainly on a 16A MCB protected circuit a 13A fuse is totally decorative.
So to acheive the same response time, if the 13a fuse is to disconnect in the same time as the circuit breaker, a lower fault reisitance is needed.
hope that helps.
regards M.