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Ok, say you had a constant operating 25 or 60kw motor in the UK. How would you wire this up? What type of MCB, fuse, starter, overload protection, wire ect would you use? Basically the rules and typical setup that you would see out on the job.
 
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Pete999

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Ok, say you had a constant operating 25 or 60kw motor in the UK. How would you wire this up? What type of MCB, fuse, starter, overload protection, wire ect would you use? Basically the rules and typical setup that you would see out on the job.
What will be the operating Voltage?
 
J

Julie.

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Most motors of this size are star-delta started, so basically you would need a ~100A star delta starter - it would have integral overload protection, then you would apply fuse (or mccb ) protection at the feed end.

Sometimes however you may specify better protection, in this case you would remove the integral protection and use something like a motor protection relay like an mpr3e5 which is much better for nps, and multi-start applications, it would just trip the starter in the same way.

Direct on line is unusual at that rating, but in practice a dol starter, or standard motor starting capacity contactor with motor protection relay, fuses/mccb on the feed to provide type c coordination.

Sometimes softstart/inverter drive is used in which case the protection is included in that, which trips the contactor similar to a relay.
 
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Most motors of this size are star-delta started, so basically you would need a ~100A star delta starter - it would have integral overload protection, then you would apply fuse (or mccb ) protection at the feed end.

Sometimes however you may specify better protection, in this case you would remove the integral protection and use something like a motor protection relay like an mpr3e5 which is much better for nps, and multi-start applications, it would just trip the starter in the same way.

Direct on line is unusual at that rating, but in practice a dol starter, or standard motor starting capacity contactor with motor protection relay, fuses/mccb on the feed to provide type c coordination.

Sometimes softstart/inverter drive is used in which case the protection is included in that, which trips the contactor similar to a relay.
How do you size the fuse/mccb before the motor protection realy? 250% of the motors full load amps? Why type C vs type D? Would across the line motor starting at least force a type D?
 
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pc1966

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Yes, DOL starting at that size is a bit brutal!

For some basic sites a fused-switch would have the means of isolation and the fault protection (say 100A for 55kW/75hp star-delta) in one place. But as Julie says you really need proper motor starter with the operational protection as well.
 
J

Julie.

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How do you size the fuse/mccb before the motor protection realy? 250% of the motors full load amps? Why type C vs type D? Would across the line motor starting at least force a type D?
It depends on the type/size of the motor; at the larger sizes we would usually be given a starting characteristic - so 2.5xFLC up to 90% of the rated speed, then we would obtain the start time from the specific application - so a star-delta having 2.5x over a 10s run-up time. In this case I would ensure the protections do not operate, and set the hot and cold times such they don't trip on start up, be do operate before the hot and cold locked rotor times respectively.

On a star-delta starter you could get away with a type B, DOL on the other hand is usually 8x flc so you would need a minimum of C - when I design a motor circuit I would try to use a type D because that gives the best chance of starting without spurious trips, if this compromises the upstream protection then I would go to type C etc.

A motor circuit is no different to any other circuit - yes it has a high starting current - but so do many lighting circuits (although smaller overall ratings).

Beyond that it's just compliance with usual stuff such as isolation, achieving suitable Zs, cable ratings, voltage drop, harmonics and RFI and so on
 
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It depends on the type/size of the motor; at the larger sizes we would usually be given a starting characteristic - so 2.5xFLC up to 90% of the rated speed, then we would obtain the start time from the specific application - so a star-delta having 2.5x over a 10s run-up time. In this case I would ensure the protections do not operate, and set the hot and cold times such they don't trip on start up, be do operate before the hot and cold locked rotor times respectively.

On a star-delta starter you could get away with a type B, DOL on the other hand is usually 8x flc so you would need a minimum of C - when I design a motor circuit I would try to use a type D because that gives the best chance of starting without spurious trips, if this compromises the upstream protection then I would go to type C etc.

A motor circuit is no different to any other circuit - yes it has a high starting current - but so do many lighting circuits (although smaller overall ratings).

Beyond that it's just compliance with usual stuff such as isolation, achieving suitable Zs, cable ratings, voltage drop, harmonics and RFI and so on

So in the UK you don't upsize the MCB to 250%, time delay fuse to 175% or none time fuse at 300%? Does BS7671 let you if you wanted to?
 
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pc1966

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So in the UK you don't upsize the MCB to 250%, time delay fuse to 175% or none time fuse at 300%? Does BS7671 let you if you wanted to?
That seems a bit arbitrary. Typically the simple approach is to look at tables of recommended combinations such as this:

Using an over-sizes OCPD is fine by our regulations as long as it meets the safe disconnection requirements for the cable, etc, and is OK for the motor protection arrangement.
Post automatically merged:

The MCB 250% / fuse 175% looks like the sort of de-rating you might need for DOL starting, but for big motors it is a bit brutal mechanically and for other users of the electric supply!
 
J

Julie.

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Arms
In general we start with the load characteristics, then we apply the appropriate protection to that, overload, fault current, etc. Then we size the cable sufficient that it is also protected by the protection, and provides sufficient current in the event of a fault such that the protection operates appropriately, is rated sufficiently to not overheat under running, starting, or fault conditions etc. Also ensure voltage drop is ok, and that's it.

No blanket add 175% or anything like that

A motor is no different to any other load, yes its characteristics are different, but the design philosophy is the same whatever the type of load.
 
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