calculation queries

[ezekiel]

when calc Zs is the end answer then compared to and need to be lower than zs for the protection device given in regs?

how is the calculation for Rcd disconnection times worked out?

is volt drop calc now based against 3% or 5% of 230v

and finally (well for now how do you know which disconnection time is used 0.4 or 5 seconds?

many thanks as usual

 

[Albert]

when calc Zs is the end answer then compared to and need to be lower than zs for the protection device given in regs?

I am not sure I understand the question; In the BS7671 2008 the max. value of the allowed Zs was reduced a bit if you compare it to the 16th. It is required to use the factor of 0.8 on the regs value to make sure that your reading complies.

is volt drop calc now based against 3% or 5% of 230v

Yes

and finally (well for now how do you know which disconnection time is used 0.4 or 5 seconds?

again not sure what the question is, I do not have the 17th book in front of me but there is a table that defines the disc. times required for TN and TT etc. configurations.

 

[GaryMo]

when calc Zs is the end answer then compared to and need to be lower than zs for the protection device given in regs?

Yes, compare your calculated Zs figure against those in BS7671:2008

how is the calculation for Rcd disconnection times worked out?

I'm not sure I understand this one, RCD disconnection time? Do you mean if the RCD trips within the required time? If so, use an RCD tester.

is volt drop calc now based against 3% or 5% of 230v

Yes, 230v is the nominal voltage used - 3% for lighting and 5% for other circuits

and finally (well for now how do you know which disconnection time is used 0.4 or 5 seconds?

TN systems. Final circuits not exceeding 32A - 0.4s, distribution circuits and final circuits exceeding 32A - 5s

TT systems. Final circuits not exceeding 32A - 0.2s, distribution circuits and final circuits exceeding 32A - 1s

 

[Albert]

how is the calculation for Rcd disconnection times worked out?

The requirement for RCD is basically calculated from 50V/30mA, this gives you the Max allowed Zs (30mA=0.03A). Probably you know that as a general requirment the RCD should be of 30mA. The 17th edition does not ask for X1 testing but for x5 and the value should not ecceed 40ms in case of earth fault.

I think that GaryMo covered the details that I missed.

Just to remind you the max Zs in the book should be multiplied by 0.8 and this figure has to be compared to your calculated or measured values.

 

[ezekiel]

yeah thanks for help the rcd questions i have trouble with are similar to:

when tested at 5 times the operating current an rcd should operate in

400ms
40ms
0.2s
0.5s

 

[GaryMo]

General purpose RCD's to BS EN 61008 or RCBO's to BS EN 61009 must trip within 300ms at 1x it's rated tripping current.
Type S within 130ms to 500ms.

5x testing within 40ms is only applicable if the RCD (not exceeding 30mA) is being used for additional protection.

With regards to the 80% figure, yes Zs figures should be compared due to the rise in conductor temperature under fault conditions having the effect of resistance in the conductor increasing. However, the 100% figures from BS7671 should be entered on the certificate.

 

[GaryMo]

yeah thanks for help the rcd questions i have trouble with are similar to:

when tested at 5 times the operating current an rcd should operate in

400ms
40ms
0.2s
0.5s

40ms

 

[Albert]

ezekiel! I see that you revising for the 17th or other exam (I might be wrong) if so you might find that the 0.8 factor might appear as a specific question.
The BS7671 Zs max values are not needed to feel-in in the original certs. Some schemes or companies that use their own forms ask for this detail.

 

[GaryMo]

Albert, do you know of any part of BS7671:2008 that specifically mentions 80% values? If not then it won't appear as part of the 17th edition 2382 exam.

 

[Albert]

Correction, I had it in my 17th exam.

 

[GaryMo]

Correction, I had it in my 17th exam.

OK then, back to the first part:

Albert, do you know of any part of BS7671:2008 that specifically mentions 80% values?

 

[securespark]

ezekiel, we're not a revising service...

 

[Albert]

The regs are guide lines, the same way if you test insulation resistance and you find it 1M ohm would you say that this is acceptable?

Most of the OSG is based on recommendations but yet we follow the guidance of this book.

 

[GaryMo]

Can you remember the wording of the question relating to 80% EFLI values?

Also, BS7671 isn't a book of guide lines but a British Standard.

 

[ezekiel]

ezekiel, we're not a revising service...

why has some1 always got to have a dig?

yes im learning, and when I dont understand I ask and usually get a great deal of help which I always thought forums were all about,

however I take your point and will keep shtum from now on

thanks for the info every1 else its helped clear up some issues for me