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

 

[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

 

[beaver74]

what type of breaker have you got protecting it B.C.D

 

[Darkwood]

what type of breaker have you got protecting it B.C.D

This is irrelevent without the knowledge of the motor size and start up characteristics, i fit all these types in different situes, its not as simple as C for motors and D for transformers so not sure where your going with this?

 

[Guitarist]

This is irrelevent without the knowledge of the motor size and start up characteristics, i fit all these types in different situes, its not as simple as C for motors and D for transformers so not sure where your going with this?

I think many of us are experienced in motors and associated control gear darkwood. As the OP hasn't been forthcoming with specifics, people are just throwing some general questions into the ring :yinyang:

 

[Knobhead]

Without some input we’re wasting out breath. Time to give up I think.

 

[djjeff99]

Right , unlike some of you on here , i dont just sit at a computer all day and night , so when i went home from work , i stopped posting . I have not just buggered off when someone asked me some questions !
Also to the ones who are trying to pull me down instead of helping me , we all have different levels of experience in certain areas .
I am a qualified electrician who has worked for years on houses and solar , but my experience on motors and industrial is limited , thats why i asked the question in the first place , if i knew it all , why would i ask ??

I will be visiting site again some time soon , so i will be able to get the info that some of the helpfull ones on here have asked for .

 

[Guitarist]

I think in general, most people here have been trying to help. We don't just sit at a computer all day and night, but do log on when we can to give advice. If someone asks a question, it is normal to expect them to keep an eye on the posts at least until that question is answered.
I was more than happy to help as much as I could, but frankly the attitude in that last post just makes me think "why bother?"

 

[Darkwood]

With all due respect Djjeff99 .... no-one has directly pulled you down or in my opinion been offensive, from the basic nature of the post its clear its not your area and as goes the my comments about no response when the correct questions asked was a dig just in general relating to many threads that stop dead when OP realising hes in too deep and at an 18hour response you have to agree that to us it was looking that way. I work 8 - 10hrs on site a day and only peruse this forum after work, all this aside maybe a Sorry for the slow response would have been a better reply rather than trying to frustrate the very ppl who have originally done their best to help you and not reading too much into the general between the lines banter.
Hopefully the air is now clear and as my previous post asked; if you can find the KW rating of the motor and explain what job it is doing, + the cable length and install method as these are really essential before any other questions are asked, prior to me asking this alot of the questions although might be relevent later in the thread may have sent you on a wild goose chase to start with.