BS 3871 Time Current Characteristics ... again!

[GB.Kayak]

Dear all, I recognise that this is a reprise of earlier queries ... and yes I have looked at the EIEMA document on the subject! I have just tested a modified lighting circuit protected by a venerable BS 3871 5 A MCB. On its side it is marked as follows, commas included as separators for clarity:


250 V, Max 5 A, M1 15/45 A M 1.5
TYPE TESTED TO BS 3871
MADE IN FRANCE RCT 40 °C


I am interpreting this to mean that it is a BS 3871 5 A type 1 MCB with a breaking capacity of 1.5 kA. Being one of the NKOTB ... I do not have either the max Z[SUB]s[/SUB] value for this or more importantly a copy of the time-current performance curves for this family of MCBs. I believe that the curves were last published in the 16th Edition of the BB; I am sorry, I didn't note which colour ...

In view of the fact that I also need to test and document a change to a 15 A radial socket circuit, and I like to do things and take responsibility for them myself, and there will be more work to do on this installation I am sure in the future, I would prefer a copy of the curves! Can any of the old(er) timers assist?


Yours Aye


GB

 

[Anthony S]

From 1985 Electrical Engineers Dairy when I was an Apprentice

15th Edition

Max Zs 12

80% as post below 9.6

 

[spark 68]

Max Zs 9.2 ohms based on 80% of the value given in the regs for a 5A type 1 MCB to BS3871

3.06 ohms as above for a 15A MCB

Type 1 curve is In X4
Type 2 " " X7
Type 3 " " X10

 

[telectrix]

deleted. usually the figure aftere the letter M is the kA rating. not sure what the 15/45 followed by M 1.5 means.

 

[Anthony S]

deleted. usually the figure aftere the letter M is the kA rating. not sure what the 15/45 followed by M 1.5 means.

deleted

 

[spark 68]

deleted. usually the figure aftere the letter M is the kA rating. not sure what the 15/45 followed by M 1.5 means.

The lower M1 might be Ics
The higher M1.5 the Icu

 

[telectrix]

so why put the kA rating of 15,20,30,45A MCBs on a 5A?

 

[spark 68]

so why put the kA rating of 15,20,30,45A MCBs on a 5A?

They could be anything from the manufacturers part/batch/model number those Tel, I was commenting on the M numbers, which as you say are the kA ratings, other than that your guess is as good as mine.

 

[Anthony S]

so why put the kA rating of 15,20,30,45A MCBs on a 5A?

I deleted my post thinking wtf am I on but the 5 A MCB is an M1 not a M1.5 likes the op says ie 1.5KA

 

[spark 68]

I deleted my post thinking wtf am I on but the 5 A MCB is an M1 not a M1.5 likes the op says ie 1.5KA

That is what I said in #6

 

[telectrix]

that's my point. i think spark68 may have the answer in post #6.

 

[Silly Sausage]

It says 'MADE IN FRANCE' so I wouldn't believe any of it.

 

[GB.Kayak]

The lower M1 might be Ics
The higher M1.5 the Icu

Thanks to all who have contributed so far ... not quite what I was hoping for, but I have clearly added to the confusion by my transcription of the inscription! I will need to check the original next week and post an update. Having read the discussion, I think that it is likely that the ratings are 1 kA for the 5 A MCB and 1.5 kA for 15/45 A MCBs though as the question has arisen in other members' minds why would the ratings for 15 - 45 A MCBs be on the side of a 5 A. I will also pull a 15 A MCB to check the text and take photos of both!

As to Ics and Icu, I am no expert ... my expertise, indeed my life, does not reach back to the 60's though conception does just ... but, my reading of the EIEMA document suggests that this terminology was introduced by BS EN 60947-2.

Expert ... ex as in hasbeen ... spurt as in leak under pressure. Quote - Ex RN Submariner 2 1/2 'ring' supply officer!

 

[spark 68]

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

 

[GB.Kayak]

Max Zs 9.2 ohms based on 80% of the value given in the regs for a 5A type 1 MCB to BS3871

3.06 ohms as above for a 15A MCB

Type 1 curve is In X4
Type 2 " " X7
Type 3 " " X10

... 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!

 

[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