BS 7671 18th edition amd 2 question?

Cazmac1 · Aug 4, 2023

Hi Guy's, I'm revising for my 18th amd 2 and came across a question that has me a little confused. I think its probably the question is badly written and I think the answer is also in correct. Can someone clear it up..

Q as written
A circuit connected to a TN system, max permissible earth fault loop impedance (ZS) for a 32 Amp socket circuit protected by a BS 88 fuse is..
A) 1.04F
B) 1.09F
C) 1.44F
D) 0.92F
Now the correct answer given in the revision paper directs me to Table 41.2 and says the answer is (A) 1.04F
Surely the F value is incorrect ???
and the question doesn't state what type of BS88 Fuse! there are two answers the highest being 0.99. So I would have rounded down and said the answer should be (D) as all other answers exceed the maximum permitted ???
If I'm being blond please be kind and explain.
Thanks

Julie. · Aug 4, 2023

Cazmac1 said:
Hi Guy's, I'm revising for my 18th amd 2 and came across a question that has me a little confused. I think its probably the question is badly written and I think the answer is also in correct. Can someone clear it up..

Q as written
A circuit connected to a TN system, max permissible earth fault loop impedance (ZS) for a 32 Amp socket circuit protected by a BS 88 fuse is..
A) 1.04F
B) 1.09F
C) 1.44F
D) 0.92F
Now the correct answer given in the revision paper directs me to Table 41.2 and says the answer is (A) 1.04F
Surely the F value is incorrect ???
and the question doesn't state what type of BS88 Fuse! there are two answers the highest being 0.99. So I would have rounded down and said the answer should be (D) as all other answers exceed the maximum permitted ???
If I'm being blond please be kind and explain.
Thanks

I don't have the latest table to hand, but from memory there is only one option, it is a final circuit on a TN system, so you know the max operating time is 0.4s. I think there is only one line for bs88 fuses (especially at 0.4s) at 32A, which from memory should be around 1.04 ohm so their answer looks correct to me.

BTW resistance is measured in ohms, F is either your mistype or the font didn't recognise the ohm symbol and substituted F

(Note you will need the bs table, not the table from the osg and read the table carefully as there may be options for bs1361 - obviously this would be 30A not 32A btw)

rewireIT · Aug 4, 2023

Where are you getting this question and its answers?

rewireIT · Aug 4, 2023

Julie. said:
I don't have the latest table to hand, but from memory there is only one option, it is a final circuit on a TN system, so you know the max operating time is 0.4s. I think there is only one line for bs88 fuses (especially at 0.4s) at 32A, which from memory should be around 1.04 ohm so their answer looks correct to me.

1.04 ohm is the answer for 30a fuses to BS 3036 according to page 61 of the blue version of the 18th.

Julie. · Aug 4, 2023

rewireIT said:
1.04 ohm is the answer for 30a fuses to BS 3036 according to page 61 of the blue version of the 18th.

OK, could be, I am not near my book at the moment, the tables changed around 2013 so I may be thinking of the older ones.

If the 1.04 ohm does line up with the old version and not the new, then it may be an outdated question applicable to an older version of the regs.

Your question about where the question comes from would appear to answer the difficulty then.

EDIT:

Just looked on-line, found this table from 2008

[ElectriciansForums.net] BS 7671 18th edition amd 2 question?

This gives 1.04 ohm for bs88 and 1.09 ohm for bs3036, so yes the question/answer appears to relate to pre 2013 regulations.

Post 2013, the additional factor of 0.95 was added, so

1.04 (pre 2013) x .95 = 0.99 ohm (post 2013 regs)
1.09 (pre 2013) x .95 = 1.04 ohm (post 2013 regs)

Pretty Mouth · Aug 4, 2023

Cazmac1 said:
Hi Guy's, I'm revising for my 18th amd 2 and came across a question that has me a little confused. I think its probably the question is badly written and I think the answer is also in correct. Can someone clear it up..

Q as written
A circuit connected to a TN system, max permissible earth fault loop impedance (ZS) for a 32 Amp socket circuit protected by a BS 88 fuse is..
A) 1.04F
B) 1.09F
C) 1.44F
D) 0.92F
Now the correct answer given in the revision paper directs me to Table 41.2 and says the answer is (A) 1.04F
Surely the F value is incorrect ???
and the question doesn't state what type of BS88 Fuse! there are two answers the highest being 0.99. So I would have rounded down and said the answer should be (D) as all other answers exceed the maximum permitted ???
If I'm being blond please be kind and explain.
Thanks

You are correct. 1.04 ohms is the 'old' max Zs for a 32A BS88-2 fuse at 0.4s, before Cmin was introduced (some time during the 17th ed). Now it is 0.99 ohms. As you say though, the Q doesn't say whether it is a BS88-2 or BS88-3.

Julie. · Aug 4, 2023

Pretty Mouth said:
You are correct. 1.04 ohms is the 'old' max Zs for a 32A BS88-2 fuse at 0.4s, before Cmin was introduced (some time during the 17th ed). Now it is 0.99 ohms. As you say though, the Q doesn't say whether it is a BS88-2 or BS88-3.

I believe the question dates from pre 2013, there was only one value given in the table (extract above), and the answer of 1.04 would have been correct at that time.

Tbh there are several websites out there which have old questions relating to previous editions, it can be counterproductive to use them, unfortunately you don't know which sites have outdated stuff until you come across issues like this.

Pretty Mouth · Aug 4, 2023

Julie. said:
I believe the question dates from pre 2013, there was only one value given in the table (extract above), and the answer of 1.04 would have been correct at that time.

Tbh there are several websites out there which have old questions relating to previous editions, it can be counterproductive to use them, unfortunately you don't know which sites have outdated stuff until you come across issues like this.

Must be pre 2011, as amendment 1 of the 17th had both listed.

Yes, I found similar errors in websites, and even some books too. It's very counter productive when you're at the learning stage and you have errors like this. You can lose hours trying to figure out how you're coming up with the 'wrong' answer, when in fact you were right all along.

rewireIT · Aug 4, 2023

At the end of the day the 18th is an exercise in finding answers in the book. Of course, it's not helpful to get the wrong answers come up on exam questions but the fact he found the info is what really matters imo

Cazmac1 · Aug 5, 2023

Pretty Mouth said:
You are correct. 1.04 ohms is the 'old' max Zs for a 32A BS88-2 fuse at 0.4s, before Cmin was introduced (some time during the 17th ed). Now it is 0.99 ohms. As you say though, the Q doesn't say whether it is a BS88-2 or BS88-3.

Spot on thanks for confirming I'll scrub that question out of my revision paper.

Cazmac1 · Aug 5, 2023

Julie. said:
I believe the question dates from pre 2013, there was only one value given in the table (extract above), and the answer of 1.04 would have been correct at that time.

Tbh there are several websites out there which have old questions relating to previous editions, it can be counterproductive to use them, unfortunately you don't know which sites have outdated stuff until you come across issues like this.

Bit annoying as I paid for past test papers advertised as 18th Amd 2. But still I was pretty sure it was an outdated question as I know my way the around book (somewhat) . Now I know the papers are not as stated I should be ok. I do have all my old Bs7671 books going back to the sixteenth so maybe I should have compared the old tables with the new.

Cazmac1 · Aug 5, 2023

Julie. said:
I don't have the latest table to hand, but from memory there is only one option, it is a final circuit on a TN system, so you know the max operating time is 0.4s. I think there is only one line for bs88 fuses (especially at 0.4s) at 32A, which from memory should be around 1.04 ohm so their answer looks correct to me.

BTW resistance is measured in ohms, F is either your mistype or the font didn't recognise the ohm symbol and substituted F

(Note you will need the bs table, not the table from the osg and read the table carefully as there may be options for bs1361 - obviously this would be 30A not 32A btw)

Hi Seems that this is an out dated question and the above would have been correct in the 17th edition. Thanks for the reply