Strange fault - larger cable tripps mcb ????? Why ???

[djjeff99]

Hi There
I have a very strange one here , that no one iv asked seems to know the answer to .

On a recent test i found a 6mm XPLE SWA cabel protected by a 63A D rated mcb , for 3 phase motor circuit .
I desided to derate the mcb to a 50A but it triipped when ther motor was running .
So i then desided to upgrade the cable to a 10mm SWA to take the 63A mcb rating .
BUT IT TRIPPED !!
Strange thing is the 63 A MCB tripps when using the 10mm cable , but not when using the 6mm cable ????
I have even tried a different brand/length of cable and the same happens .
Everyone in the game i ask has never come accross this happening before , so i thought id give it a try on here ? :dizzy2:

Thanks
Jeff

 

[telectrix]

the lower resistance of the 10mm may be allowing a higher start up current to flow.

 

[Engineer54]

Put your amp clamp on these cables and see what this motor is pulling, as the very first step in your fault diagnosis...

 

[Smudge]

should it not be the o/l unit on the starter that trips first for overcurrent, not the mcb? Have you ensured any of the integrity of the terminal studs/cable (or whatever) has not been damaged/inside the motor connection box. The motor could just be on its way out with an insulation fault? Looking at possible reasons for mcb tripping...

 

[djjeff99]

I will put the amp clamp on to see what exactly its pulling on start up , and i dont think it has a starter on it , but maybe it needs one .

 

[djjeff99]

the lower resistance of the 10mm may be allowing a higher start up current to flow.

Thanks
Interesting point , yea could be .
What options do i have to stop this tripping ?

 

[Guitarist]

Over 0.37kw it should have a starter and O/L. Is it just running off a 3-pole switch at the mo?

 

[djjeff99]

Over 0.37kw it should have a starter and O/L. Is it just running off a 3-pole switch at the mo?

Yes it is !
Thanks for the info .

 

[Engineer54]

I will put the amp clamp on to see what exactly its pulling on start up , and i dont think it has a starter on it , but maybe it needs one .

Errr, ...Yes i think it needs one too, ...in fact it most definitely needs a starter control point!!! lol!!!

 

[Guitarist]

Also, check whether it is wired as star or delta configuration so you can read the correct rating off the motor plate, then set your o/l to this.
I would do a full insulation/resistance test on the motor before anything else tbh. Tripping an mcb with a much higher rating than the motor sounds more like a fault than overload if you are using a D type. As suggested, if it meggers out ok, stick a clamp meter on each phase as it could even be unbalanced.

 

[Darkwood]

A lot of shots in the dark here.... the first question is what size Kw is the motor and is it a star/delta motor?, how is it connected if its as suggested 'Direct on Line', what is the length of cable run to the motor and installation method.
Answer these questions and im pretty sure i can tell you why and what and how to solve it.
Mcb tripping etc means nothing and dosn't imply faulty motor until you know what size Mcb should be covering this motor so first thing first before everyone confuses you with random ideas..... What size is the motor and what is it doing as a workload.

 

[Knobhead]

As darkwood has said without more information we’ll just be guessing. As well as the motor details what sort of drive is it?

 

[Darkwood]

As darkwood has said without more information we’ll just be guessing. As well as the motor details what sort of drive is it?

What drive??? has a VSD been mentioned???

 

[Engineer54]

What drive??? has a VSD been mentioned???

I think he means the likes of, is it pully or direct coupled rather than VSD drive ...lol!!!

 

[Knobhead]

A fan or pump is different to start than say a conveyor. Sorry I should have said load.

 

[Darkwood]

A fan or pump is different to start than say a conveyor. Sorry I should have said load.

Soz Tony, i was mislead as you already pointed out what id posted which included the motor workload, so assumed you didnt mean that by the way you worded it as been a point not yet mentioned but hey ho'!, Same old - same old; OP disappears as soon as the right questions are asked lol

 

[Knobhead]

As per usual!

But to be honest he seems out of his depth when it got to starters and overloads.

 

[Guitarist]

The company I worked for several years ago employed a firm of electricians to help us on-site guys install and connect up several new pieces of equipment as time was of the essence. A few bits of kit had no control cabinets at all so we had to design and build from scratch. We were amazed to find that none of the electricians had ever heard of a star/delta starter, let alone ELV control circuits. A few hours later they had been briefed in the art of reading a basic circuit diagram and the properties of contactors. As far as management was concerned we were all electricians so where was the problem....

 

[Knobhead]

“It’s only three wires, where’s the problem” to quote one of our engineering managers.

 

[Jonesy83]

i had this argument with someone who is supposedly a time served industrial electrician who could not read a drawing to wire up a very basic roller shutter motor starter

i was a 1st year apprenice at the time and did the job whilest my boss was sleeping off a hangover in the van