Earth Fault Impedence

Can anyone please explain what is the maximum earth fault loop impedance permitted for moulded case circuit breaker (MCCB)?

Also, BS 7671 says a disconnection time of 5 sec is allowed for breakers in distribution circuits. What is the difference between a distribution circuit and final circuit?

I don't know.

are you really in South Carolina?

why the interest in BS?

I will update my location, it was by mistake...

Table 41.3 of BS 7671 provides Zs for upto 125A breakers only. I need the loop impedences for larger breakers (150A, 200A, 300A, etc). Where can I find Zs for the larger breakers?

Loyal1 said:

I will update my location, it was by mistake...

Table 41.3 of BS 7671 provides Zs for upto 125A breakers only. I need the loop impedences for larger breakers (150A, 200A, 300A, etc). Where can I find Zs for the larger breakers?

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You need manufacturers data - as you've been told on a number of forums now.(hardly living up to your name )

It isn't just a case of 'I've got a 300 A MCCB, what's the Zs?'

You need something like this:

http://www.schneider-electric.com.h...at_filenames_I-lineMCCBsPanelboard_SQD001.pdf

"Electrifying" link is really good, wish they all were that good! For B, C, and D rated MCB's there is no need to look in Big Red Book although in their efforts to help it has in some ways made it harder to understand. It states for B type 46/In and multiply 46 x 5 and you get 230 the same for a D with 11.5 x 20 also 230 so it is simply B = 5x rated amps C = 10x rated amps and D = 20x rated amps for Instantaneous ratting. So for a 230v supply using ohms law for example a B16 MCB you will need 80A to Instantaneous trip and 230/80 = 2.875 ohms.

However as "Electrifying" states with larger moulded breakers the same magnet parts are used in a range of breaker sizes and his link shows both 16 and 20 amp have same magnet trip.

The main reason for using moulded breakers rather than MCB's is the short circuit capacity jumps from 10kA to 25kA well some MCB's can be as low as 4.7kA this means unless the earth loop impedance is less than 0.05 we would not really want to use such expensive switch gear.

Before we get anywhere near 0.05 the short circuit capacity will have increased anyway and we start looking at 35kA and above.

The next point is the relationship between PSC and ELI if one divides 230 by PSC one gets the ELI however this is not strictly true as ELI always is measured to earth and PSC is measured both to earth and to neutral and the highest figure is taken. With a TN-C-S supply likely they will be the same but of course with a TT supply they can be leagues apart.

With parallel paths and smaller earth to neutral cables as we leave the supply head PSC and ELI will lose their relationship and measurements will no longer show ELI as 230/PSC yet on many test reports that is exactly what one sees. It is so obvious what has been done. The meter has been connected and ELI taken and then the selector changed to PSC and that reading also read off. Seaman's tester I used always showed as 240/PSC and I often wondered which was the accurate measurement ELI or PSC as we did not have a uniform 240v throughout the site. And as one looked back through the records to detect any trends it was quite obvious that the electricians back for 15 years had all failed to take two readings for PSC and there was a sudden jump where I had started to take highest reading.

It was also obvious that the readings had been taken and written down but no one had ever sat down and worked out if they passed! I had to fit a number of RCD units to comply with ELI readings being too high. And since these had not changed in years likely the readings were too high when originally installed. I was lucky the electrician had left. But with other jobs I have had the problem of finding a fault which must have existed for years and to try to get it corrected without dropping another electrician in it is hard. Bosses don't like one saying Fred missed this fault 5 times over the last 15 years!

A distribution circuit feeds another board which will have overload devices in, where a final circuit feeds direct to an appliance/machine or socket outlet.

The main reason for using moulded breakers rather than MCB's is the short circuit capacity jumps from 10kA to 25kA well some MCB's can be as low as 4.7kA this means unless the earth loop impedance is less than 0.05 we would not really want to use such expensive switch gear.

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Afaict another reason for using low value MCCBs is because you want to take a critical circuit from as close to the origin as possible to minimise the risk of power loss on it (e.g. fire alarm supplies, lift supplies etc)

Loyal1 said:

Also, BS 7671 says a disconnection time of 5 sec is allowed for breakers in distribution circuits. What is the difference between a distribution circuit and final circuit?

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The definitions are in the 2nd section of the regs, basically a distribution circuit is for the distribution of power to other boards. For example a submain from the main CU to a garage CU etc is classed as a distribution circuit.
A final circuit is a circuit which connects to current using equipment, or an accessory for the connection of (socket outlet etc).

Thanks all for good information. I have one basic question. What is the purpose of measuring earth fault loop impedance? It makes sense to measure earth fault loop impedance if the protective device is earth leakage breaker or RCD; but why measure the earth fault loop impedance for regular circuit breakers or fuses?

Because the EFLI needs to be low enough to ensure the OCPD will operate within the maximum permitted disconnection time in the event of a short circuit from live to earth.

The cable resistance values (R1+R2) are readily available for smaller cables. However for cables larger than 25mm2, we have to consider the cable inductance as well. Does anybody know a table which shows the reactance of cables larger than 25mm2?

4D1B, 4D2B, 4D3B......

Thanks for help ... If I have a circuit breaker with a frame size of 100A and a trip of 32A, then shall I have to use the Zs values for 100A or 32A?

Standard MCBs will only have one max. figure for it's given rating/type

Loyal1 said:

What is the difference between a distribution circuit and final circuit?

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Loyal1 said:

why measure the earth fault loop impedance for regular circuit breakers or fuses?

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Loyal1 said:

Does anybody know a table which shows the reactance of cables larger than 25mm2?

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(In other words I don't have a copy of the Wiring Regulations.)

Loyal1 said:

If I have a circuit breaker with a frame size of 100A and a trip of 32A, then shall I have to use the Zs values for 100A or 32A?

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Loyal1 said:

I need the loop impedences for larger breakers (150A, 200A, 300A, etc).

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I have a bad feeling about all of this.

No, actually I am talking about MCCB. If I have a MCCB with a frame size of 100A and a trip of 32A, then shall I have to use the Zs values for 100A or 32A?