2392 theory exam: maximum earth loop impedance

[louisIV]

I've got my 2392 theory exam this evening. Am looking through a mock paper which has this question:

What is the maximum earth fault loop impedance allowed when using a Type C circuit breaker giving a disconnection time of 5 seconds? The nominal voltage to earth is 55V.

Is there a way to calculate this (ie to get the answer without looking up the table in the Regs book)?

Thanks,

LIV

 

[septiclecky]

I've got my 2392 theory exam this evening. Am looking through a mock paper which has this question:What is the maximum earth fault loop impedance allowed when using a Type C circuit breaker giving a disconnection time of 5 seconds? The nominal voltage to earth is 55V.

Is there a way to calculate this (ie to get the answer without looking up the table in the Regs book)?

Thanks,

LIV

Look in the regs book

 

[louisIV]

My tutor told me that the exam is closed book - can't take the regs book in. Which is why I'm wondering if there's a calculation method.

 

[a1]

I think he meant have a look in the regs book now, not in your exam.

 

[spinlondon]

5.5 divided by the rating of the MCB.

So a 6A MCB would be 5.5 divided by 6 = 1.92 when corrected to 2 decimal places.

 

[louisIV]

I mis-copied the original question. It gave the value of the breaker (32A). Sorry for that. I can't really memorise all the maximum Zs values for all the breakers and fuses, which is why I was wondering about calculation.

Spinlondon - thank you. I'm not sure why I work with 5.5 rather than 55, but your formula allowed me to get to the correct answer in the paper.

Doesn't seem to work for another similar question in another paper which simply says,

The maximum earth loop impedance to BS 7671:2008 for a circuit protected by a 20A BS88 fuse giving a 5 second disconnection time is:

a) 7.42ohms

B) 2.91ohms

c) 1.84ohms

d) 1.35ohms

This one doesn't even give a nominal voltage. Am I expected to just know all the values on tables 41.3 to 41.4?

---------- AUTO MERGE Post added at 14:18 ---------- Previous post was at 14:11 ----------

... And also know Table 41.6 (which covers 55V) off by heart?

 

[spinlondon]

This applies to all BS EN 60898 MCBs.

For Type C MCBs, you divide the voltage by 10 and then divide by the MCB rating. Type B MCBs you double the result, and Type D you halve the result.

So if to had a 230V 6A Type B, 230/10 = 23, 23/6 = 3.83, 3.83x 2 = 7.666, round it up to 2 decimal places and you get 7.67.

Hope this helps.

 

[louisIV]

This applies to all BS EN 60898 MCBs.For Type C MCBs, you divide the voltage by 10 and then divide by the MCB rating. Type B MCBs you double the result, and Type D you halve the result.

So if to had a 230V 6A Type B, 230/10 = 23, 23/6 = 3.83, 3.83x 2 = 7.666, round it up to 2 decimal places and you get 7.67.

Hope this helps.

This is great. Nobody has ever mentioned this in my course, or as yet on my 2330 Level 3. Thanks so much!

LIV

 

[spinlondon]

Would that be similar to magnetic trip settings?

 

[septiclecky]

You could also use thr magbetic trip settings to work it outType B 6 amp (Type B 3-5 In) always use the greater of the two figures

5In x 6amp = 30 230volts/30=7.67

6 Amp Type C = (5-10IN) So 10 x 6 = 60 230/60 = 3.83

Thats for 230 v he has 110v CTE

 

[m4tty]

Plumbers version is the way I way taught in my coursework.

Just curious LouisIV. Is there a practical part to the 2392 as ive done the theory exam and passed but never done the practical and was never told about it if there is one. Thanks

 

[sprocketflup]

Matty - yes there is. Or there was when I did it anyway

 

[seajayess]

Multiply MCB tripping current by rating factor ie. Type B X 5 Type C X 10 Type D X 20

Divide 230 Volts by result

Then multiply by 0.8 for correction factor.

B 6 Amp 60898 MCB

6X5=30

230/30=7.67

7.67 X 0.8= 6.14 ohms

Maximum Zs 6.14 ohms corrected

Hope this helps CJS

 

[Larnacaman]

That's a New One on me!!!!!

Correction factor for What ??? :coffee

 

[sprocketflup]

I think he is on about that you are suposed to be within 80% of the tabulated values in the BRB larnacaman

 

[spinlondon]

You have to allow for the inrease in resistance of the conductors when they are at operating temperatures. The figure given in BS7671 are given for an ambient temperature of 10 degrees C, wheras the operating temperature is 70 degrees C.

You multiply the figure given in BS7671 by 0.8 and that will give you your maximum calculated or measured values.

 

[Larnacaman]

70C is Maximum temperature for PVC cables, ambient always used to be quoted at 30C.

So if they are now so concerned about adding a correction factor, why don't they quote the 70C rating as the Maximum earth loop impedance for a given breaker size and type??

So what, there's another correction factor for XLPE insulated cable??

 

[Chr!s]

70C is Maximum temperature for PVC cables, ambient always used to be quoted at 30C.So if they are now so concerned about adding a correction factor, why don't they quote the 70C rating as the Maximum earth loop impedance for a given breaker size and type?

The maximum Zs are for operating temps of 70C, and the ambient temp in appedix 4 is 30C.

 

[Larnacaman]

No offence ...just in one of those Sarky moods today .... lol!!!

 

[Chr!s]

No offence ...just in one of those Sarky moods today .... lol!!!

No probs

The 0.8 is just a factor applied when carrying out the test due to cable temp being much lower than the 70C, when designing the 0.8 is omitted.