Selecting the correct circuit breaker 1

[cha1n]

Hi,

I am a trainee control systems engineer and i'm currently designing a panel and i would like to know how to select the correct circuit breaker for my project. I spoke with a colleague in work and was horrified to find out that they guess the correct size and expect me to do the same.

The motor i require a CB for is 3 phase (415VAC), direct on-line and 2kW.

I also require a CB for a 24VDC, 5A PSU.

Some terms that i have seen online whilst trying to research this subject include; "line current", "load current", "FLA", "Inrush current", "power factor" and "efficiency (losses". So if any can clear these up too that would be great!

Any help is greatly appreciated, i'm based in the UK, but feel free to reference to 'NEC' as i've seen in other threads, i believe the uk use 'IEEE' is that correct?

Regards,

Ross

 

[FreddyNurk]

As a (somewhat) experienced electrical engineer, it concerns me that it appears that companies all over are neglecting to train and mentor up and coming engineers, and I suspect this is a prime example.

The compliance with relevant standards depends on the locale, and the particular standard being referenced, but I'd say its safe to say that for most electrical applications, the referenced standard is law.

Designers, installers and so on thus have an obligation to ensure that whatever they design, commission, install and so on complies with the relevant standard, and thus the onus is on them to provide the relevant standard. I do not believe it is possible to state that something is designed to a standard without having the ability to refer to the standard when required.

If your employer doesn't have access to the standards, or worse, refuses to provide them, then unfortunately its probably better for you if you look elsewhere. Observing mistakes can sometimes be a great way to learn, but in these applications to embark on such a process is at best foolhardy, and at worse likely that you'll end up in court. Rest assured that any company that feels that it can avoid its duty to design to an appropriate standard is not likely to think twice about providing a scapegoat when (not if) the proverbial excrement hits the air propulsion device.

 

[cha1n]

My question is how do you know that a standard exists for a given application? How do you define the correct standards to use and how do find out if you're required by law to follow them? *** This is the most important question ***

I've only just joined the company and it's a very small one, they currently contract thier electrical work to a self employed electrical engineer and they've not once mentioned standards. I can't imagine the bosses reaction when i say "can you please buy me this £300 standard?", especially if there are multple standards to purchase (which there obviously are). If i'm going to ask such a question i definately want to know whether said standard is required to be complied to by law or otherwise i'll just get a straight "NO" as an answer to my request.

Prior to this company i did an apprenticeship at General Electric and i definately remembered the engineers there mentioning standards and directives.

Thank you all for your replies.

 

[sendoUK]

Any electrical installations must comply to IEE regs BS7671:2008 (the wiring regs) at the least. Ideally your company (or the electrician you sub it out to) would be NICEIC registered, and probably should have done his Part P given the size of jobs he does.

You can buy the regs from anywhere such as WH Smith or from Amazon (regs + on site guide, which would be useful for someone like yourself, about £70)

 

[cha1n]

Hey,

I just called the British standards enquiries line to ask them about that area. They said that british standards are a guide, but when following a regulation they are required by law.

So my new task is identifying regulations to follow. So as you said there are the wiring regulations (BS7671:2008), but how would i find out any other regulations to be followed? Also, within the BS7671:2008 regulations, will they reference to other british standards that must be followed (therefore making them compulsary by law)?

I'm starting to think that a) I should get the company to buy a copy of BS7671:2008 and b) Is it compulsary to have completed the City and guilds 17th edition to design electrical installations.

Any ideas, people i can call, etc?

Ross

 

[ScottyUK]

sendo,

As I noted previously the BS 7671 wiring regs apply to installations, not to equipment. The most relevant standard for equipment is BS EN 60947. Part P is irrelevant for a non-domestic installation.

cha1n,

In answer to your question, it is not technically a legal requirement to conform to the relevant Briitsh Standards, but by doing so you 'usually' do meet your legal obligations although it is not implicit in the standard. If you decide to ignore the standards and it goes wrong then the prosecution's case will probably focus on why you, as a competent person, believed that you knew better than the authors of the standard when you designed whatever it was that caused the incident. It's a pretty much indefensible position - much easier to comply with the standards!

Have a look at

http://www.schneider-electric.co.uk

You'll have to register to be able to download some information, but search for 'DIA1ED2061002EN' in the search field. This document gives some information which I think might help you understand the subject a little more and which contains some equipment combinations which will be suitable for 'typical' applications near the back.


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If we learn from our mistakes I'm getting a great education!

 

[cha1n]

ScottyUK,

Firstly, thanks for the reply. You mention that BS EN 60947 is the most relevant standard for equipment. If in future i wanted to find the correct standard to follow for my particular application, how can i find out?

I was about to order a copy of BS 7671 and i assumed that as it was the regulations for wiring that it would contain details on sizing CBs etc, but it seems like you're suggesting that CB sizing guidelines are found in a seperate BS, do i understand correctly?

Thanks

 

[ScottyUK]

BS 7671 deals with sizing circuit breakers for protection of an installation, e.g. thermal protection of cables, maximum fault clearance times, etc, and provides some detailed tables of cable ratings in different installation conditions on which it is possible to select an appropriate breaker or fuse. It's undoubtedly a very good standard, but doesn't directly apply to your problem other than the supply which feeds it. It's worth having anyway if you do any installation work, and the On-Site Guide mentioned by SendoUK is invaluable especially on domestic and light commercial jobs.

BS EN 60947 is titled Low Voltage Switchgear and Controlgear. It's a big standard with a lot of parts (i.e. it is f'ing expensive) and like most of the British Standards, including BS 7671, it tells you what you must achieve but not how to achieve it. The engineer's job is ensuring that his design meets the standard, and part of that is being able to interpret the standard and apply it to the real world. The manufacturers have a vested interest in selling equipment which either complies with, or is easy to design into a system which complies with, a standard. That's why I suggested the Schneider website as a starting point. Siemens have some good guide material on their site too, as do others.

The cheapest way of finding standards is time spent looking on BSI's website or occasionally the IEC equivalent using the search engine. There's a standard for almost everything if you look hard enough. Some are pretty obscure to say the least.

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If we learn from our mistakes I'm getting a great education!

 

[cha1n]

Is there some sort of table that let's you know if a BS has IEC equivalent? If there is, which one do you work to?!

I feel i'm digressing from the topic, but another issue i've just encountered is wire colours. Does the wiring regs state correct wire colour for all voltages?

I'm trying to find out if there are set cable colours for 24VDC wiring and for field and local wiring to PLC I/O Cards.

Cheers,

Ross

 

[LionelHutz]

Hey Scotty

Is the design work to be done by a professional engineer?

Is there an equivalent in the UK to UL508A - which is a panel shop listing. We are a registered UL508A panel shop which allows us to build industrial control panels and apply a UL label. This has a UL standard and random UL inspections to ensure we are meeting the standard.

 

[ScottyUK]

Lionel,

Yes, I would expect it to be designed by a professional engineer. No, there's not a direct equivalent to UL508A as far as I'm aware, although there probably should be. My impression is that (some of) the US standards are more practically oriented than the European equivalents. Some of our standards are written by lawyers for lawyers, or that's how it feels.


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If we learn from our mistakes I'm getting a great education!