Earthing and fuse question....

Dazzac · Oct 17, 2009

Hello all

I have a question that is really bugging me. I have searched the internet but cannot find an answer so hopefully someone here can help so here goes.......

It is to do with earth fault loop impedence. If a line conductor touches an earthed part say a washing machine, then the fault current passes through the earth conductor to the electode if TT or pme for TNC-S etc.

Question....

So we have a 32A MCB on the circuit where the fault has occurred. How does the fuse blow under these conditions if the fault is carried down to earth? How does the current increase enough for the fuse to blow? If the current is flowing to earth how can a fuse blow if a high percentage of that current is not passing through the fuse? And also the earth is totally separate from the actual conductors so I can't understand how the fuse actually blows! I hope I haven't confused things too much!! Please correct me if am wrong, i am still learning so bear with me!! Cheers Daz

roukel01 · Oct 17, 2009

The MCB is protecting the circuit at the consumer unit. An earth fault occurs (Low Resistnce) somewhere in the circuit, which in turn creates an increase in current passing through the MCB to the fault, down the earth, down the PME or whatever is protecting it, to the supply transformer, back through the windings of the transformer and back to the CU, which makes the MCB break. Thats the complete Earth "Circuit" or loop, which is what you're testing when you do a Zs reading.

Darkwood · Oct 17, 2009

As roukel has already explained above why the fuse trips but i think your confusion arises because you still think that the current flow will just change its path and remain the same current flow but this isnt the case, due to the very low resistance normally associated with a short to earth for a split second before the fuse operates thousands of amps can flow which as in your case is plenty enough to operate the fuse within the specified time the regs give you.

Graeme Harrold · Oct 17, 2009

darkwood said:
As roukel has already explained above why the fuse trips but i think your confusion arises because you still think that the current flow will just change its path and remain the same current flow but this isnt the case, due to the very low resistance normally associated with a short to earth for a split second before the fuse operates thousands of amps can flow which as in your case is plenty enough to operate the fuse within the specified time the regs give you.

Your PSSC and PFC will provide your highest fault current expected in an installation under fault conditions. The deeper you go into an installation, the higher the resistance path and thus the lower the fault current, which is where your Zs readings become more critical. If your Zs exceeds the maximum permitted, the current flowing for a given fault will not be enough to trip your protective device within the necessary time.

Have a look at the time curves in the BRB to start understanding the operation of them and the minimum current required to make them trip. Then have a look at the current they can carry and times taken before tripping if you get it wrong

Darkwood · Oct 17, 2009

Graeme Harrold said:
Your PSSC and PFC will provide your highest fault current expected in an installation under fault conditions. The deeper you go into an installation, the higher the resistance path and thus the lower the fault current, which is where your Zs readings become more critical. If your Zs exceeds the maximum permitted, the current flowing for a given fault will not be enough to trip your protective device within the necessary time.

Have a look at the time curves in the BRB to start understanding the operation of them and the minimum current required to make them trip. Then have a look at the current they can carry and times taken before tripping if you get it wrong

I wasnt been ignorant of the info youve supplied was just simplifying the answer and i did use the the terms 'usually' and 'can', because of the type of question he asked it was clear he just learning the trade or just a curious enthusiast' so i didnt want to blind a simple answer with Zs, PSSC, PFC, just in case he didnt understand them.
I was just giving an off the cuff example as to what trips the mcb during a fault and why.
Hope you didnt think i was stealing your thunder as that wasnt my intension i was simply writing the answer for someone who may not have electrical knowledge (which i dont know if he has)
Oh! Just to confirm when i said "i think your confusion arises" it was directed at daz and not yourself as from the answer you gave it obvious i dont need to teach you to suck eggs

Graeme Harrold · Oct 17, 2009

darkwood said:
I wasnt been ignorant of the info youve supplied was just simplifying the answer and i did use the the terms 'usually' and 'can', because of the type of question he asked it was clear he just learning the trade or just a curious enthusiast' so i didnt want to blind a simple answer with Zs, PSSC, PFC, just in case he didnt understand them.
I was just giving an off the cuff example as to what trips the mcb during a fault and why.
Hope you didnt think i was stealing your thunder as that wasnt my intension i was simply writing the answer for someone who may not have electrical knowledge (which i dont know if he has)
Oh! Just to confirm when i said "i think your confusion arises" it was directed at daz and not yourself as from the answer you gave it obvious i dont need to teach you to suck eggs

OOOps, wasnt meant as having a go, just used the "Quote" rather than "Reply" button. I got the same feeling that the OP's knowledge is in the early learning stages as he does not have a clear vision of a circuit under fault conditions and the working mechanics of an MCB. in that the current will 99.9% of the time flow through the MCB.............

robsparky1975 · Oct 17, 2009

There are some good answers on the forum which all point you in the right direction,hope i can add something to help,we all have ways of explaining things differently,so here goes.

A circuit breaker has 2 ways of operation:
1) Thermal: for overload/short circuit.
2) magnetic:for earth fault currents

when you have a fault to earth on a circuit like that the fault current will pass down the circuit protective conductor(earth inside a cable) down the supply authority earth(tns or tncs)to the local substation and return to the source of fault,this is the earth fault path.a fault with a low enough earth fault loop impedence (zs)reading will trip the magnetic field of the circuit breaker,the lower the reading the better,so as to be within maximum limits permitted for these devices to operate within a specified period of time,the fault current can be generally 1ka(1000amps)as a rough guide most circuit breakers can withstand 6ka(6000 amps).
if for instance your circuit had an earth fault loop impedence reading of 0.60 ohms (zs) at that point,the magnetic part of the circuit breaker would trip very quickly,under 0.4seconds,as required for that type of circuit, BS7671 shows the maximum zs values for all types of protective devices(fuses,circuit breakers etc),with a graph chart showing the relation of fault current/time.

The lower the zs value the quicker the protective device operates.

The higher the zs value the longer it will take protective device to operate.