BS 3871 Time Current Characteristics ... again!

[spark 68]

Don't worry so much about the 1 or 1.5 kA breaking capacity, if it is in a Single phase CU protected by a 100A or less fuse to BS1361 type II then they have a conditional rating of 16kA as part of that type tested assembly.

 

[GB.Kayak]

Don't worry so much about the 1 or 1.5 kA breaking capacity, if it is in a Single phase CU protected by a 100A or less fuse to BS1361 type II then they have a conditional rating of 16kA as part of that type tested assembly.

I am not worried on the electrical front ... I just like to know what nomenclature means for when it does matter. I should have worked my way back to the supply which is in another building to check what the protection is at the origin of the installation. This sub-main is just a switched distribution board.

 

[GB.Kayak]

Ics is the max fault current it can safely switch, and still be serviceable.

Icu or sometimes Icn is the max fault current it can safely break, but not be re-usable

I already gave you the trip curves earlier for the 0.1 -5 secs disconnection times

EIEMA document identifies the 'M' category rating as being a BS 3871 concept which disappears in BS EN 60898; in its place the Rated Short Circuit Capacity or Icn.

 

[spark 68]

... so is that 0.8 x 12 = 9.6 Ohms or 9.2 Ohms? Whichever way, with a Z[SUB]e[/SUB] or according to GN3 Z[SUB]db[/SUB] as this is a sub-main of 0.19 Ohms and an R1 + R2 of 0.39 Ohms, Z[SUB]s[/SUB] for the lighting circuit is well inside at 0.58 Ohms ... safe as houses! I won't be recommending a new board yet then!

the full formula for determining max Zs for fixed curve MCBs to either BSEN 60898 or BS 3871 is

(Uo/IA)X 0.8

Uo = 230V

IA is the tripping current required to open the breaker in 0.1 - 5 secs, this is the instantaneous part of the curve and is In X the type multiplier

So if we work the first part out we have for a 5 A MCB type 1 MCB

In = 5 Amp
Type 1 multiplier is X4

230/(5X4) = 230/20 = 11.5 which is known as the tabulated value as given in the regs

now we need to multiply this by 80% to take into account of the temperature correction of the cable at 70 degrees, this then gives us the max Zs that we need to ensure that our measured result is below

so 11.5 X 0.8 = 9.2 ohms = max Zs

we normally compare to 80% of the regs tabulated max Zs, ie lower than the tables given in BS7671 when we are testing.

 

[GB.Kayak]

the full formula for determining max Zs for fixed curve MCBs to either BSEN 60898 or BS 3871 is

(Uo/IA)X 0.8

Uo = 230V

IA is the tripping current required to open the breaker in 0.1 - 5 secs, this is the instantaneous part of the curve and is In X the type multiplier

So if we work the first part out we have for a 5 A MCB type 1 MCB

In = 5 Amp
Type 1 multiplier is X4

230/(5X4) = 230/20 = 11.5 which is known as the tabulated value as given in the regs

now we need to multiply this by 80% to take into account of the temperature correction of the cable at 70 degrees, this then gives us the max Zs that we need to ensure that our measured result is below

so 11.5 X 0.8 = 9.2 ohms = max Zs

we normally compare to 80% of the regs tabulated max Zs, ie lower than the tables given in BS7671 when we are testing.

Thank you for your derivation ... Anthony S appears to have rounded to 12 Ohms, hence the difference between your accurate answer and his approximation. Yours has been a useful prompt which reminds me of my course lecturer running through this as one approach to deriving max Zs on my course.

 

[spark 68]

Thank you for your derivation ... Anthony S appears to have rounded to 12 Ohms, hence the difference between your accurate answer and his approximation. Yours has been a useful prompt which reminds me of my course lecturer running through this as one approach to deriving max Zs on my course.

If Anthony was quoting from a 15th edition document then 240V would have been used in that calculation to get that figure, I have not checked it yet

When we come to amd3 we need to further lower the Zs by a further 5% due to the new C min factor of 95%

 

[GB.Kayak]

If Anthony was quoting from a 15th edition document then 240V would have been used in that calculation to get that figure, I have not checked it yet

When we come to amd3 we need to further lower the Zs by a further 95% due to the new C min factor

Thanks, that would make sense! I noted the additional 5 % reduction in an earlier thread ... maybe I need to change allegiance ... RAeS to IET to get the discount on the books!

... still can't justify a board change ... ;-((

 

[spark 68]

or just use 0.76 (about 3/4 of the tabulated values) in place of the 0.8 for temperature, 0.95 X 0.8 = 0.76 , this will account for both then.

Edit:unless they have already lowered them in the BYB, hmm I will have to check.

 

[GB.Kayak]

or just use 0.76 (about 3/4 of the tabulated values) in place of the 0.8 for temperature, 0.95 X 0.8 = 0.76 , this will account for both then.

Edit:unless they have already lowered them in the BYB, hmm I will have to check.

So 68 ... age or year of birth ... ;-P

 

[spark 68]

So 68 ... age or year of birth ... ;-P

Age mate lol, I tell you what this bloody IE 11 browser is giving me grief on here, that and the lager lol

 

[Silly Sausage]

or just use 0.76 (about 3/4 of the tabulated values) in place of the 0.8 for temperature, 0.95 X 0.8 = 0.76 , this will account for both then.

Edit:unless they have already lowered them in the BYB, hmm I will have to check.

So that's where your 0.76 comes from (the missing bracket post! )
I didn't have the balls to ask at the time.


I still think it somewhat strange that you can have a formula for max Z esses.

 

[Silly Sausage]

Age mate lol, I tell you what this bloody IE 11 browser is giving me grief on here, that and the lager lol

Not IQ then?!

Try Opera Opera browser - The fast, safe web browser - Download free

 

[spark 68]

So that's where your 0.76 comes from (the missing bracket post! )
I didn't have the balls to ask at the time.


I still think it somewhat strange that you can have a formula for max Z esses.

Yep, I use it all the time, I rarely look it up in the book for fixed MCBs, I usually just calculate it on the spot if needed, for fuses then I need to look up, apart from common domestic ones, cos I have a printed sheet lol

I will have to try that 0.76 calc out a bit more first to make sure it works properly before I adopt that method fully.

 

[GB.Kayak]

Age mate lol, I tell you what this bloody IE 11 browser is giving me grief on here, that and the lager lol

Safari for me on the iPad! My problem is sufficient 3G bandwidth ... I think that either the storms have affected the local masts or there has been a significant rise in 3G usage due to higher gadget density following Christmas ... or 3 are throttling my usage! Probably all 3!

 

[spark 68]

The forum software does not play nice with IE 11, it misses words/letters out, I have to type so sloooow I might try fire fox and see if that is any better.