Zs and BS88 fuses

[Chri5]

I'm trying to work out what the maximum Zs should be on some circuits that are fed with BS88 fuses. The OSG has tables on page 101 which is all well and good but some of the circuits are using the SWA as the CPC. Can somebody give me a clear explanation how to arrive at a maximum value in this circumstance.

Cheers

 

[Evans Electric]

I don't think it makes any difference Chris, measure the Zs , note the fuse size and compare it to the tables pp48 - 49 BRB . What disc. time is it for ?

If its within the scope that should be fine .

Or is this not an actual job ????

 

[Technician]

Go with Evans... page 48/49 red book. (in the amended version there are

some changes).

Table 41.2 in the green book has disc time for BS88-2 and -3 page 55.

Table 41.4 has the disc times for BS88-2 and -3 page 56.

There are some changes in value from the red to the amended version.

Check disconnection time.

Find allowable Zs for that disc. time, apply Rule of thumb to tabulated value of 0.8.

i.e. tabulated Zs value multiplied by 0.8 as per appx 14.

If the SWA is used as a CPC then you may need to use the adiabatic to determine

suitability of armouring as CPC. To do this you need a prospective fault current

value.

 

[Chri5]

Firstly yes this is an actual job.

I see your point Evans and I've pretty much decided to use those values but I still don't get why the OSG is listing Zs values according to the size of the cpc.

Can anybody explain that to me please.

 

[Technician]

Chri5; the OSG values may be different because the

rule of thumb may already have been applied.

I do not have the OSG to hand right now but in other

publications the Rule of Thumb (0.8) has already been

applied to the values given in the regs.

There are some reasons for not applying the Rule of Thumb

and there is more than one of these rules. This is why the

values in the regs are uncorrected.

Can you give me the page and the version of the OSG you

have?

 

[Chri5]

I'm on page 101 of the red OSG. The tables are set out differently to the ones for MCB's and I really don't know why. The ones for MCB's don't mention the size of the cpc at all. I thought the rule was that the cpc had to be at least half the size of the line conductors. If this is satisfied and the Zs value is within limits for the particular fuse then surely the circuit can be deemed safe.

 

[Technician]

Chri5;....Have looked at that table.

If you divide the values on page 48/49 Regs

by the value in that table you get a ratio

of 0.8. This is the rule of thumb I mentioned

earlier.

This means that the rule of thumb has already

been applied so the maximum permissible value

of Zs has been corrected for fault conditions.

Remember that the amended version AND the

new OSG are out. Use these for the future.

Read the note in small print at the bottom of page

49. It refers to appx 14.

 

[Chri5]

Are you telling me I've got to buy yet another regs and OSG?

 

[Technician]

Chri5...read the top of page 7 in the amended version when you

get it. It comes into force January 1 next year. This means

that installations completed after that date must comply with the

new amended version.

Awfully sorry chum.

 

[Chri5]

Man its nothing more than a bloody money making scam. 60 quid yet again. Is there any place you can download the amendments?

 

[Technician]

Chri5;...I think there is...give me a little time and

I may have that for you.

The changes to allowable values of Zs will not be

in there though. I think I should forewarn you about

that.

All the best in the meantime. Hope my posts assist.

 

[Sidewinder]

There are more changed pages in the green book than there are pages the same as the red book, plus there are sections moved etc. so there is no "update" to the red book, you'll have to have a completely new green book.

The IET do an electronic version, though there are no legitimate free downloads available of any of the IET books.

 

[Technician]

Further to Sidewinder... have a doc on the principal amendments

BUT the revised allowable Zs values (pages 48/49 Old) are NOT Listed.

 

[Chri5]

So really it's the 18th edition in disguise. Either that or they didn't do a proper job the first time round. Now we all have to pay 60 quid for their errors. Oh well it's keeping somebody in a job I suppose. Anybody want to by a red regs book?

 

[Nicky Tesla]

borrow the guide off somebody and rewrite or copy the relevant changes yourself.

What does it matter if the number system has changed as long as the work done is compliant.

 

[Chri5]

Not a bad idea

 

[steptoe]

tell me,

would you write out the whole of the regs book for 60quid?

how long do you reckon it would take you to copy it out?

and how long would it take you to earn

 

[Sidewinder]

Learn to live with regs updates guys Amd 2 is already in process.

There will be another Amd about every 2 years probably for the life of the 17th due to globalisation.

Just learn to factor it into your business overheads just like all the other costs you incur when running a legitimate business.

 

[Chri5]

i like writing



---------- Post Auto-Merged at 19:23 ---------- Previous post was made at 19:22 ----------

There will be another Amd about every 2 years probably for the life of the 17th due to globalisation.

What does that mean? Is BS7671 being adopted throughout the world?

 

[Sidewinder]

No, it means that due to the advancement of technology and changes in the requirements for electrical installations that IEC 60364 (IIRC) will be updated and member countries to the IEC will then have to update their national standards to reflect the changes in the IEC Std.