Electrical earth fault loop impedance calculation

studio54

Hi

Just wondered if anyone can help - I'm sitting an exam on Monday and wondered if this solution to a past exam question was right. More importantly, part three asks what effect fitting an RCD on design Calculations would have but I'm not sure.

Any suggestions? (See attachment)

Cheers

2011

Here are a few pointers:

Once you know the type of circuit you know the required disconnection time. Armed with this and knowing the rating of the protective device the table gives you the maximum earth fault loop impedance value.

I think that it is best to use just one unit (reduces the chances of an error).
In this case I am using Ω rather than mΩ.

The earth fault loop impedance, Zs = Ze + (R1 + R2 )
I do not see this equation on your attachment.

In your case you are given Ze = 0.8 Ω (not 0.8 mΩ)
This value will not change.


For every meter of the circuit the increase in R1 + R2 = 0.00889 + 0.01452 = 0.02341 Ω

The RCD does not effect the earth fault loop impedance.

Risteard81

As above, but the question RE: the RCD might be about how it alters what Zs will ensure that disconnection times are still met even if the protective device wouldn't disconnect in the required time.

studio54

Thanks for responses!

Does that mean that the RCD also has an impedance????

I know that this is something specific to supplying socket outlets, I just am not sure what?

2011

studio54 wrote: »

Does that mean that the RCD also has an impedance????

Everything has impedance, RCDs, fuses, MCBs etc...

However the negligible impedance of the RCD is not taken into account in questions such as this.

I think that mentioning the RCD confuses the issue.
The point is that socket circuits require a different (faster) disconnection time.
This is not because they have an RCD, it is because they would be perceived has having a higher risk (e.g. than a cooker circuit).

Risteard81

Not about internal impedance of the RCD but the fact that a 30mA RCD (0.03A) will disconnect if the circuit has a higher impedance. To operate where touch voltage exceeds 50V this would be 50/0.03

studio54

2011 wrote: »

Everything has impedance, RCDs, fuses, MCBs etc...


The point is that socket circuits require a different (faster) disconnection time.
This is not because they have an RCD, it is because they would be perceived has having a higher risk (e.g. than a cooker circuit).

.... But what is the "higher risk"? Is it that something plugged in to sockets is "mobile" as opposed to "fixed", for example an electric lawn mower and potentially seen as more of a hazard - mower cuts lead?

2011

studio54 wrote: »

.... But what is the "higher risk"? Is it that something plugged in to sockets is "mobile" as opposed to "fixed", for example an electric lawn mower and potentially seen as more of a hazard - mower cuts lead?

Regardless of how well an installation is wired and tested the electrician has no control over what will be plugged into a circuit at a future date.

So a perfectly wired socket circuit could have a dangerous appliance plugged in.
Compare that to a cooker: A competent person / electrician should only connect up a cooker that is safe.

2011

I think that many electricians and apprentices struggle t understand earth fault loop impedance, specifically:

⦁ Why is it so important?

⦁ What has this to do with breaking capacity?

⦁ Why is the breaking capacity of MV switchgear so much lower than LV switchgear?

⦁ Why does protective device with a larger rating require a lower earth fault loop impedance value?

⦁ How does this impact on disconnection time?

⦁ Why does a 20A C type MCB require a lower earth fault loop impedance value than a 20A B type MCB?

In my opinion it is easier to "get" earth fault loop impedance when you understand the answers to the above questions.

studio54

Well thought out advice.....Very much appreciated

Thanks

2011

@ studio54:
I think that your attachment is being misunderstood.

Part (b) is broken into three parts, (i), (ii) and (iii).

Look at (iii):

(iii) If the circuit was used to provide supply to socket outlets, in what ways would the design be affected? (20%)

Someone has typed the following answer:

(iii) For socket outlets of up to 20 Amps an RCD would be required.

Note: (iii) is shown in red for the answer.



I think that the answer being looked for was:

The disconnection time should be reduced from 5 seconds to 0.4 seconds. This would require a lower earth fault loop impedance value