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Topic Title: PSC
Topic Summary: Live conductor CSA
Created On: 05 July 2013 02:24 PM
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 05 July 2013 02:24 PM
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zeeper

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Should we/me be taking into account PSC when selecting CSA of Live conductors.
 05 July 2013 05:48 PM
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OMS

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Yes, of course you should - both under earth fault and short circuit conditions

Try regulations 433.1.1 generally and specifically 434.5 et seq (particularly 434.5.2)

Regards

OMS

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Failure is always an option
 06 July 2013 09:11 AM
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zeeper

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My interest is in High PSC and the size of live conductors.

Thank you for the reg numbers for reference.

Please feel free to correct me if Im wrong , but it appears that the regs requirement is for the conductors to with-stand maximum I2t of the protective device.
 07 July 2013 03:13 PM
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AJJewsbury

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it appears that the regs requirement is for the conductors to with-stand maximum I2t of the protective device.

I think that's right. There is a 'deemed to comply' thing for standard' devices - so long as Iz>= In, and the overload protective device has a suitable breaking capacity, then fault protection is assured (reg 435.1 2nd & 3rd para) . If course that doesn't necessarily help if you have reduced c.s.a. c.p.c. or live conductors that's aren't directly protected from overload by the protective device (e.g. ring circuits) - so with high PFC and MCBs you might start to notice problems with rings wired in T&E....
- Andy.
 07 July 2013 09:53 PM
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zeeper

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Thanks for the response AJ.

We have transformers on-site,so on some sub boards PFC can get up towards 10KA.

Due to up coming works to be carried by contractors I was reminding myself of the adiabatic. As I not a fan of twin and earth being used on my site, I thought I better arm myself.

Anyway I then got to think about PSC and live conductors.
 08 July 2013 02:03 PM
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AJJewsbury

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You might already know (so just in case..) - if you're using MCBs, there's a useful set of tables in annexe ZA of BS EN 60898 which gives maximum I2t values for different types & ratings of breakers - so given a value for k it's easy enough to compare with conductor size. Manufacturer's data may well have lower values but if you want keep the ability to replace protective devices in the future without problems it might be worth insisting on the generic BS EN values.
- Andy.
 10 July 2013 01:42 PM
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zeeper

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BS EN 60898


Thanks you the pointer but I dont hanve access to that document.

So i'll have to rely on finding the manufacturers data.
 10 July 2013 02:07 PM
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OMS

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Class limits are set in Annex ZA for type B and C breakers in a range of nominal current ratings for stated fault levels.

BS EN 60898 and being a Class 3 devices will have (for Type B), energy limitation of:

3.0kA - 18000A2S
4.5kA - 32000A2S
6.0kA - 45000A2S
10kA - 90000A2S

This is in a range for MCB's (Type B) of 20A, 25A and 32A

Regards

OMS

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Failure is always an option
 10 July 2013 02:09 PM
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AJJewsbury

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I dont hanve access to that document.

Briefly, it says, for class 3 MCBs, the maximum permissible energy let-though is:

For devices In <= 16A:
3kA rated: B-type: 15,000 A²s, C-type: 18,000 A²s
4.5kA rated: B-type: 25,000 A²s, C-type: 30,000 A²s
6kA rated: B-type: 35,000 A²s, C-type: 42,000 A²s
10kA rated: B-type: 70,000 A²s, C-type: 84,000 A²s

For devices 16A < In <= 32A:
3kA rated: B-type: 18,000 A²s, C-type: 22,000 A²s
4.5kA rated: B-type: 32,000 A²s, C-type: 39,000 A²s
6kA rated: B-type: 45,000 A²s, C-type: 55,000 A²s
10kA rated: B-type: 90,000 A²s, C-type: 110,000 A²s

with a note that 40A breakers should have maximum values of 120% of the above.

it also gives (higher) values for class 2, and no limits at all for class 1. but I think most MCBs these days are class 3.

- Andy.
 10 July 2013 02:11 PM
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zeeper

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maybe you can help me with the numbers

I have been looking at Za1 and Za2 (merlin gerin c60)

looking at a 10KA type B class 3 upto 16A MCB we get a I2t of 70 000.

So if s = I2t sqrt/k,

s=70 000 sqrt/115,

264.6/115 = 2.3mmsq

Is that right
 10 July 2013 02:11 PM
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AJJewsbury

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Although OMS types quicker than me
- Andy.
 10 July 2013 02:15 PM
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AJJewsbury

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maybe you can help me with the numbers

I have been looking at Za1 and Za2 (merlin gerin c60)

looking at a 10KA type B class 3 upto 16A MCB we get a I2t of 70 000.

So if s = I2t sqrt/k,

s=70 000 sqrt/115,

264.6/115 = 2.3mmsq

Looks about right to me - as a check 115² x 2.3² (i.e. k²S²) = 69,960.25 = within rounding errors of 70,000.

- Andy.
 10 July 2013 03:05 PM
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OMS

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Originally posted by: AJJewsbury

Although OMS types quicker than me

- Andy.


Copy and Paste Andy -


OMS

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 10 July 2013 04:01 PM
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zeeper

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So is this information telling me that if this route was used to select conductor sizes in stead of" In ".

I would need a minimum sized conducotrs of 2.3mmsq for PFC protection. For say a 10K 6A type b class 3 device.
 10 July 2013 04:25 PM
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AJJewsbury

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So is this information telling me that if this route was used to select conductor sizes in stead of" In ".

I would need a minimum sized conducotrs of 2.3mmsq for PFC protection. For say a 10K 6A type b class 3 device.


I think so, yes! (which is why I pointed out the handy 'deemed to comply' bit above).

It does make you think about the wisdom of using 1.5mm2 c.p.c. on a RFC with a 32A MCB on a notionally 16kA supply - remember that MCBs are now built to EN standards and our continental cousins tend to have a higher impedance supply and wouldn't dream of putting anything less than a 4mm2 c.p.c. on a 32A device (no reduced c.p.c.s and no rings).

I suspect that it's been a problem lurking for a while - a bit like 1.0mm2 c.p.c. on old 2.5mm2 T&E - but it's gradually being noticed and quietly put right. GN 3 now contains the table from the OSG which lists minimum c.p.c. sizes for MCBs in addition to the max Zs values.

- Andy.
 10 July 2013 05:16 PM
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OMS

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Perhaps also worth pointing out that with a difference between line and CPC conductor size, it's usually the smaller CPC that exhibits the highest temperature rise - but may well be allowed to get to a higher temperature during fault

That said, I doubt very much you have 10kA fault levels present - when you take into account both power factor (of the fault) and the current limiting effects of upstream protection.

It needs checking, yes - but only if you are really expecting a close up fault (ie at the DB end) - you don't need many metres of small cable to drop the fault level dramatically

Regards

OMS

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Failure is always an option
 11 July 2013 07:51 AM
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zeeper

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We do have some sub boards in the 5kA region ,and at least one i know of getting on onwards 10kA.

I'm just trying to develop a basic thought process for assessing the minimum size of conductors with reference to PFC.

My current logic now being If the live conductors meet the requirement of " In " they are ok for PFC.

And if the magnitude of fault current is greater than I can read off against the protective device curve in app3 then I cant use adiabatic. I have to use 543.1.4. to for fill the requirement of bs7671.
 11 July 2013 09:58 AM
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OMS

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Generally, yes - I'd go along with that.

Regards

OMS

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Failure is always an option
 11 July 2013 10:25 AM
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zeeper

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If I have a fault current 1200A on a 32A B type , could I not just as a rule of thumb go

1200x1200x0.1sqrt/115

I understand that it would may be quicker than 0.1 but with no other data would it be reasonable to do so.
 11 July 2013 11:22 AM
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AJJewsbury

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And if the magnitude of fault current is greater than I can read off against the protective device curve in app3 then I cant use adiabatic. I have to use 543.1.4. to for fill the requirement of bs7671.

In my head you can still use the adiabatic - S = sqrt(I²t)/k is the same as k²S² = I²t - just that it's re-arranged and you get I²t from device data rather than t. Using I²t rather than t means you don't have an actual value for t alone - which makes sense with MCBs and the like as the time taken to open will depend to some extent on where in the a.c. cycle the fault starts, even though the overall energy let-through would be the same. Appendix 3 is 'informative' rather than 'normative', so there's no requirement to use that data if you can find better data elsewhere (e.g. the standard for MCBs or manufacturer's data).

If I have a fault current 1200A on a 32A B type , could I not just as a rule of thumb go

1200x1200x0.1sqrt/115

I understand that it would may be quicker than 0.1 but with no other data would it be reasonable to do so.

You could, but 0.1s is damn slow for an MCB at those sorts of currents - so you're building in a very large (and so maybe costly) safety margin. I²t for 1,200A for 0.1s is 144,000 A²s - compared with a maximum of 18,000 A²s allowed by the standard - e.g. for k=115, 3.3mm² rather than than 1.17mm² - and the margin will increase for larger currents.

- Andy.
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