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

 

[jraef]

Ross, No offese, but...

This is the type of work that should be done by a qualified ELECTRICAL engineer. The fact that you do not know the meanings of the terms you mention indicates that you are not such, and the practice of your co-workers of "guessing" is a fire waiting for a place and time to happen.

Breaker selection needs to be done by first knowing the installation codes of where it is going to be used. Saying that NEC or IEEE is OK for use in the UK is another indicator that you should not be undertaking this task. Please look for and contract with a qualified EE in your area on this.

ScottyUK where are you? This guy needs help!


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[cha1n]

jraef,

I understand where you are coming from, but i need to learn how to do this regardless.

I am a trainee, so that's why i need help. The company has an electrical engineer contractor that they employ, but he is away for 2 weeks and if i don't find the correct way of selecting a CB they are going to guess the rating and i don't want them to do that.

I am trained in Electronic and Electrical engineering (only to HND level), but the bias was heavily towards electronics, i covered only a small section on transformer efficiency and electrical circuit design.

I know there's alot of talented people on here, hopefully someone can help!

Ross

 

[dpc]

A lot of this is driven by local codes as much as by engineering.

We have quite a few engineers in the UK who contribute to this forum, so stick around.

BTW, NEC is National Electrical Code - basic installation code in the US. Nothing to do with IEEE.

 

[cha1n]

Hey dpc,

I looked at the NEC website earlier today and it seems very good, but i can't find anything like it for the uk unfortunately..

I understand that codes could influence things, but surely things like "FLA", "power factor" and "efficiency (losses)" apply to all countries?

To be honest, this is an issue regarding work and i finished nearly 4 hours ago, so i should be getting on with my university coursework, but like i said i'm very keen to find out the answer to this because if noone helps i know i'll be over-ruled and they will just 'guesstimate' a CB rating!

 

[jeebusmn]

Mike Holt's website has a pretty good forum. I don't know how much it will help because you are in the UK but they have a lot of electricians that frequent it. I agree with the others that it is a little disconcerning that you don't know what codes you are supposed to follow. You might want to check out the IEC.

http://forums.mikeholt.com/

http://forums.mikeholt.com/archive/index.php/t-52420.html

 

[cha1n]

Thank you for your reply.

I agree with you, it concerns me that i don't know which codes to follow, but i'm very willing to learn and i don't think that my degree course is going to let me know either..

I'm starting to wonder who's responsibility it is to inform students which codes to follow to be honest!

Ross

 

[Truemanator]

FLA - Full Load Amps. Refers to the maximum current draw from a load. For example on a motor (I'm a 4th year power EE, so experts correct me please), the FLA will be the rated current.

Load current - current drawn by an impeding load. Can be complex or completely real load.

Line current - Current flowing on a supply line. For example if you're operating three phase, each phase will have its own, possibly different, current flowing.

Inrush current - the current it takes to get an inductive coil energized over the first few milliseconds. Very short in duration but very important for protection. Similar to starting current on a motor (where the starting current is needed to get the motor up to rated speed).

Power factor - Ratio of real power (P, Watts) to active power (S, Volt-Ameres). pf = cos(P/S). S = P + jQ. Q being reactive power in VAr. pf is a complex topic, ask if you need more information.

 

[cha1n]

Thanks for another useful post!

Believe it or not the power factor looks the most familiar! Is it usually referred to in percentage then? I remember someone saying to me that usually a PF of 0.8 is used, so i assume that's 80% of real power?

Cheers!

 

[dpc]

I'm sure you can find a good explanation of power factor via a Google search. And you must have an ac circuits textbook tucked away somewhere.

 

[Truemanator]

Oops I mistyped that. Power factor is pf = cos(phi), where phi is the angle in electrical degrees between the current and the voltage waveforms. pf = P/S as well.

Yes, power factor is always less than or equal to one. It is NOT a percentage, though. Do not assume that if pf = 0.8 that it's 80% real power and 20% reactive power (very wrong). Take the sin(phi) and you'll see what I mean. pf = 0.8 yields a reactive factor of 0.6.

Also, you may see "leading" or "lagging" power factor. This is in reference to the current sine wave leading ahead or lagging behind the voltage sine wave.

There's lots of information out there on this. I only began to understand it after a lot of reading.

 

[Truemanator]

Keep in mind all those current definitions are sorta industry lingo. Truth is, V ALWAYS equals IZ. Always. Never ever forget that. You can solve for current on any conductor by taking the voltage drop and dividing by the load/line impedence.

 

[jraef]

The facts remain that;

a) the proper breaker selection has more to do with local fire codes than the specifics of design as you are getting into here, and ...

b) getting it wrong is risking a fire.

Breaker selection is about nothing more than protection against overloading of conductors; both those feeding the equipment and those inside of it (such as motor or transformer windings). Internal equipment conductors will be based on nameplate data on the equipment, so you usually base the feeder conductors off of the same data. So using that, FIRST you select your conductors based on the worst case load scenario your equipment will see, THEN you select the breaker that protects them. The rules for making those selections vary by region / country / state / maybe even city codes.

The reason the NEC says nothing about the UK is that NEC stands for National Electrical Code, with "National" meaning the Nation of the United States of America. It applies absolutely NOWHERE else. It doesn't even apply to Canada, they use the CEC, or Canadian Electric Code. Get the point? You need to find out what the governing installation codes are for the UK. Absolutely nothing else is meaningful to your situation. What is legal and acceptable in Turkey or China or Idontgiveahootstan may be completely illegal in the UK. If you select a protective device based on the wrong codes, it may have an effect on other devices within the facility that are following the code, leaving your installation in a vulnerable position or possibly causing a dangerous condition somewhere else.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[raptor6458]

cha1n, jraef is right. this is all about safety!
whether its human , property, etc. you still have to follow the existing code of your country and locality. since you are in UK i believe you should be using the IEC.
and yes, scottyUK can sure help you.
and remember " a little knowledge is a dangerous thing"!!!

 

[mgtrp]

Although not intimately familiar with British electrical codes, I'd guess that the wiring rules (BS 7671) would be more relevant than IEC standards.

 

[ScottyUK]

For breaker size I will take an educated guess in the absence of information and say a 10A 3-phase breaker would be a likely result of the calculation, but there is more than just current rating to consider. You need to know about the inrush and locked rotor currents of the motor and be sure that it doesn't trip the breaker when the contactor first closes. You have given no details of where this motor derives its power from, so this breaker could end up being anything from a 10A type C MCB capable of breaking a fault not exceeding 6kA through to a MCCB capable of clearing an 80kA fault. At present I simply couldn't tell you.

As a general rule you should not be using a CB to protect a motor: normally it is there to protect the cabling and to provide backup short circuit protection. There are breakers designed for motor duty but they are less common. Your motor should have a proper thermal overload relay to protect it, and again without knowing anything about the load it could be a Class 10, 15, 20 or 30 O/L relay. Class 10 is most common over here but it depends on the requirements of the load. You should have a contactor switching the motor, which I hope you have.

BS 7671 is the UK wiring regulations for fixed installations. The standard which is most relevant to your task is probably BS EN 60947. If you want an easier read than the BS then (e.g.) the Siemens and Telemecanique catalogues both have a lot of useful selection guides which will probably help you. They're much easier going than the standard, but the standard is what you need to comply with.

Have a look at the modular motor starters available from the bigger manufacturers, which give you a breaker, contactor and overload in a type-tested combination. They are getting quite common for small loads and can give you an easy design solution if your load and motor is close to what the manufacturer used as their 'typical' parameters.

Please get someone to check your design before it gets built.


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

 

[cha1n]

Wow, thank you for all of the replies.

I'm sure about the source of the 415VAC 3phase supply to the motor. All my panel will do is switch it on and off using a digital output on a siemens PLC (via a contactor).

The CB is there for the installers to wire in/out to and all the information i have on the motor i've stated (Direct On-line, 2kW, 415VAC 3Phase).

ScottyUK: The standards that you are referring to, are they available to free download? Are we in the UK obligated by law to abide by the standards?

Do the standards contain all definitive information on selecting CBs, cable size, etc, etc.

I've struggled with things that people who design panels must think of simple tasks, such as how to layout the control panel. I can't find any books or information on this area, but it must be documented somewhere.

Thank you for your time people.

Ross

 

[cha1n]

I meant "I'm not sure about the source of the 415VAC" and would you say i need additional information to select a CB. A colleague mentioned a MPCB, would that be the type of CB to use?

 

[sendoUK]

cha1n:

I'm slightly alarmed that the company you work for would just 'guess' at a suitable breaker size to protect a circuit. No-one but a trained Electrical Engineer should be doing this. By asking you to do this without training they are asking you to break the law as I see it - if there was a fire as a result of an incorrect breaker being fitted and somebody died, we are talking corporate manslaughter here.

It's fine if you are training, and you are being asked to do this so that you can learn, so long as somebody competant is checking your work before any installations take place.

That aside, there is more info required here. Firstly, the panel you are feeding the motor from - what type is it, MCCB panelboard? MCB Dist Board? Fuse board? MCC Panel? Then there's the cable supply to the motor - the breaker you install must be suffiently rated to not only not trip when the motor starts, but to trip in time to protect the cable and prevent overload on the circuit.

Then there are many other considerations - what capacity do you have on your electrical system to add extra circuits? Will the breaker you install achieve discrimination with any upstream devices? Is it suitably rated for the fault level that it is likely to encounter?

In very simple terms, a 2kW motor at 415V (if it really is 415V and not 400V as it should be in the UK, although it makes little difference at this low a load) will draw about 5A running current, although the starting current could be much higher, and your breaker will have to be suitably sized to clear the starting current, or else suitably in-sensitive so as not to trip as soon as an overload is detected.

 

[cha1n]

Hi SendoUK.

I have to admit i was told that the supply was 415VAC by someone else, i've not been able to confirm it myself (yet).

The panel is a control panel containing a Siemens PLC and whatever additional parts are required for controlling the motor(s). As far as i can see it will connect like so:

3 Phase wired into CB > Contactor (PLC Digital output controlled) > Motor

Does that seem correct? The panel is also receiving 4-20mA signals from various pressure transmitters and temperature sensors which as also fed to the PLC and viewed via a HMI. There are 2 motors being controlled (2KW) and 2 Pumps (1 lobe and 1 hot water supply; 2KW and 3KW respectively).

I've recently found out that i need to design an emergency stop circuit for the panel, i wont bring that into this topic properly because unfortunately i've discovered that it's also a complicated subject (sigh).

I've identified the standards required for the E-Stop ciruit and others have stated the standards regarding circuit breakers, but how do i get access to them? Is it free of charge? Does every uk business have to abide by them because i just don't think my company (it's a small one) even uses them, but i really want to make sure i do if it's required by law.

Thanks again people.