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Topic Title: Replacing Switchgear and
Topic Summary: falling foul of disconnection times
Created On: 27 December 2017 06:02 PM
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 27 December 2017 06:02 PM
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nad

Posts: 410
Joined: 14 January 2005

Season's Greetings One & All,

Got a little issue with the replacement of a Motor Control Centre.

Am replacing a motor circuit's switch-fuse, motor overload and motor starter.

The original install was pre17th and had to only meet a 5s disconnection time but now with the 0.4s rule for all 32A circuits and under, the installation does not meet today's max Zs.

I could juke the stats and install a 40A switch-fuse but the motor manufacturer recommends a 32A fuse for this particular set-up.

And the Client does not want to pay to up scale the cable which is currently about 70m of 2.5mmsq SWA.

Does anybody have any advice? I'm particularly interested in how best to certify this work?

All the Best,

Nad

-------------------------
Nad

*Regularly edited due to spell cheque misdiagnosis
 27 December 2017 06:05 PM
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nad

Posts: 410
Joined: 14 January 2005

Best add, the motor overload is roughly set to about 20A

-------------------------
Nad

*Regularly edited due to spell cheque misdiagnosis
 27 December 2017 06:39 PM
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Nedryerson

Posts: 126
Joined: 12 December 2009

NAD,

Whats the motor rating ?

Assume 3c SWA?

Ned
 27 December 2017 06:53 PM
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nad

Posts: 410
Joined: 14 January 2005

How do Ned,

I'm not back in the office till the new year and don't remember the exact details but I'm fairly sure the motors 7.5kW.

I've not check the cable myself yet but have been told it's 5core, with 2cores used for control.

-------------------------
Nad

*Regularly edited due to spell cheque misdiagnosis
 27 December 2017 08:43 PM
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leckie

Posts: 4522
Joined: 21 November 2008

When you cannot meet the disconnection times you require additional protection. So I think you can provide his by supplementary bonding.
 27 December 2017 08:47 PM
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Nedryerson

Posts: 126
Joined: 12 December 2009

NAD,

According to my calcs the 2.5mm will provide the required disconnection time

To be sure what's the fault level ?

Volt drop is way over but the motor will likely tolerate that.

Ned
 27 December 2017 10:08 PM
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nad

Posts: 410
Joined: 14 January 2005

Ned,

Unfortunately I don't have any test results yet but I've run the circuit in amtech, with a good estimate of Zdb, and the R1+R2 is the only significant contributor to Zs. Hopefully it will pass but I doubt it.

Now if we leave the circuit it's OK as it complied with the regs at the time of install. But what is the accepted practise when the switchgear is replace but not the cabling. My argument is it's not any worse than when I found it and all I can do is recommend that it is brought up to code.

-------------------------
Nad

*Regularly edited due to spell cheque misdiagnosis
 28 December 2017 03:05 AM
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leckie

Posts: 4522
Joined: 21 November 2008

Well as I said you could look at 411.3.2.6.

We don't have enough information to carry out the cable calculation but if volt drop is also an issue, and also possibly the thermal protection of the cable under fault conditions, there might be grounds for thinking that the cable may not have complied even to an earlier edition of the regulations.

If the circuit was designed and complied with an earlier edition of the regulations but not the current edition in every respect, BS7671 advises us that the installation may not necessarily be unsafe for continued use or require upgrading. But this is not to say that it necessarily is considered safe for continued use and does not require upgrading either; that's a judgement call. There are lots of things that complied with a previous edition that are no longer considered safe and do require upgrading.
 28 December 2017 07:25 AM
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Jaymack

Posts: 5393
Joined: 07 April 2004

Originally posted by: nad
The original install was pre17th and had to only meet a 5s disconnection time but now with the 0.4s rule for all 32A circuits and under, the installation does not meet today's max Zs.

Does the MCC installation require conformance to BS7671?

Regards
 28 December 2017 09:51 AM
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leckie

Posts: 4522
Joined: 21 November 2008

Or fit an an RCD to provide fault protection? So it could be 300mA, 100mA,..
 28 December 2017 10:04 AM
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AJJewsbury

Posts: 16178
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Am replacing a motor circuit's switch-fuse, motor overload and motor starter.

Is it direct like-for-like replacements (e.g. because the originals have been damaged) or ones with different ratings (e.g. because of a larger motor)?

If the former I might be tempted to treat it like a repair and just note on the certificate that the circuit doesn't meet current standards for disconnection times. If however you're in effect creating a new circuit, but just re-using some handy existing cables, I'd be more comfortable if it complies fully.

One option might be to use an RCD of some kind to allow the required disconnection time - it doesn't have to be a twitchy 30mA unit - even a 500mA time delayed one or an adjustable earth leakage relay should be capable of opening well within 400ms when faced with a L-PE fault of negligible impedance.

- Andy.
 28 December 2017 10:06 AM
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AJJewsbury

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Ah, leckie got there first!
 28 December 2017 11:23 AM
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nad

Posts: 410
Joined: 14 January 2005

Good morning All, Thank you for your posts.

This is a factory where downtime is a dirtyword. The old MCC is being replaced as the new one will be safer to maintain live, the control hardware & software will be upgraded as well, but the existing cabling reused and the field equipment will stay the same.

The customer wants a test certificate for the new panel. I am fairly certain they would rather replace the cable than install RCDs or sup bonds but this could take months to organise. And in the meantime I was wondering what peoples views would be on how best to certificate the new panel?

The move from the 16th ed fixed equipment 5s disconnection times to 0.4s for what seems to be an arbitrary 32A rule had not caused me an issue until now. I agree with the aims of this in make us all safer but wondered how others had dealt with this type of situation? If this was a new consumer unit in a home, would I be able to provide the customer with an installation certificate?

-------------------------
Nad

*Regularly edited due to spell cheque misdiagnosis
 28 December 2017 01:44 PM
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leckie

Posts: 4522
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So basically you think the Zs is now likely be too high but you don't want do anything to change things? Well I can see what you are saying about it not being any worse that it is at present.

In your situation I would not be happy to work on equipment from a supply that was not in my opinion safe. So if the volt drop limits where exceeded that would be one factor. If the Zs is too high and a fault took longer to clear then I would want to check if the cable was adequate in terms of thermal withstand. If it wasn't I would say that it is not suitable and would be wanting to make the necessary alteration to make it comply. I cannot see why there would be an objection to supplementary bonding, this could probably be done with very little down time as most of it could be prepared before hand. You could also install a new cable beforehand and just switch off for the connections. You are going to have some down time to sort the motor controls out so other than monetary concerns I can't really think of any reasonable objections to not sorting out the problem.

If the cable is OK in terms of of VD and thermal withstand, the addition of supplementary bonding may have a knock on effect in terms of lowering the Zs value depending what the bonding entails; although this is not the objective of the supplementary bonding it might be an additional benefit. It also a cheap and easy method in most cases.

Regarding a certificate, what type are you going to issue, an EIC? I suppose you could carefully describe to extent of the work being carried out and comment on the existing installation. But I wouldn't feel too happy mentioning that the supply is not adequate in terms of VD, thermal withstand of the cable under fault conditions, and exceeding the maximum disconnection time permitted. Others might disagree with me on that of course, it's down to your own judgement of the actual situation and if you think it is safe. We don't really know because we don't have adequate information.

Good luck with it anyway Nad.
 28 December 2017 03:53 PM
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Nedryerson

Posts: 126
Joined: 12 December 2009

Nad,

From tables the R1R2 for a 70 metres 3 core SWA is 0.98 ohms

From tables again max Zs for 32A BS 88 fuse is 1.04 ohms However I wonder if the 5 core has more armour and hence a lower R1R2 ?

For example, from DRAKA the maximum armour resistance for a 2.5mm 3 core is 8.2 ohms/km whereas 5 core 2.5mm is 6.8 ohms/km a 20 % decrease which might just get you over the finish line provided the source impedance isn't excessive.

Also, are you sure the cable is 70 metres long?

Will they let you stop the motor to measure the loop ? If not remeasure the cable.

Ned
 28 December 2017 11:07 PM
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Weirdbeard2

Posts: 126
Joined: 29 November 2017

Originally posted by: nad

Unfortunately I don't have any test results yet but I've run the circuit in amtech, with a good estimate of Zdb, and the R1+R2 is the only significant contributor to Zs. Hopefully it will pass but I doubt it.



Hi Nad, what kind of fuses are you proposing? Motor rated or general?
 02 January 2018 05:51 PM
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nad

Posts: 410
Joined: 14 January 2005

Good evening All, Thank you very much for the replies.
WB2, they are general fuses. the fuses, etc. are as specified by our control panel designer and as equipment manufacturers recommendations.
Ned, I've not got test results yet and probably won't until the works shutdown. I do have an estimate for the cable length.
Leckie, all other parameters pass in an amtech model. and perhaps the circuit will pass with the preexisting parallel paths. Ideally I want to issue an EIC upon completion but doing so with a circuit missing from the schedule and a caveat stating it misses its disconnection time is a bit pants.

-------------------------
Nad

*Regularly edited due to spell cheque misdiagnosis
 02 January 2018 06:53 PM
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Weirdbeard2

Posts: 126
Joined: 29 November 2017

Thanks Nad, further question, why are you replacing the switch fuse, and where is it on the circuit?
 02 January 2018 08:06 PM
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ToniSM

Posts: 346
Joined: 21 November 2006

I'm somewhat confused and concerned about this thread.

The OP hasn't been to the job and therefore can't provide any meaningful figures?

My holistic helmet is away for recalibration so I'm ducking out of the thread.

-------------------------
Could there be a better way?

In theory yes, but in practice?
 02 January 2018 08:25 PM
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Weirdbeard2

Posts: 126
Joined: 29 November 2017

Originally posted by: ToniSM

I'm somewhat confused and concerned about this thread.



The OP hasn't been to the job and therefore can't provide any meaningful figures?



My holistic helmet is away for recalibration so I'm ducking out of the thread.


Hi Toni, agreed but so far we have a 7.5 KW motor, overload set about 20A, no one sure of anything but it's better than watching coronation street / emmerdale with the Mrs
IET » Wiring and the regulations » Replacing Switchgear and

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