One to ponder....

[Darkwood]

Got a new customer and in short I wired up an old lathe to its new position, on commisioning I noted that th old electrics were dated and had illegal additions...

The motor run was a physical switch that just energised the coil of the contactor direct.
The E-stop just interupted the switch

So a power failure and power return would start the lathe without warning and also closing the E-stop would also start the lathe back up ... that aside when testing the Estop it was reset and because the lathe wasn't in Neutral for starting it took out the mcb because it was DOL on load.....thats the background and my field so easy fix for me, the strange thing is when the TP Merlin mcb tripped it also tripped the adjacent DP rcd unit .....

Discuss why! (ps the rcd fed sockets and no earth leakage was the cause of its trip)

PS I know the reason but wondered if anybody can come up with the theory of why a mcb tripping for known reasons tripped an adjacent rcd unit.

 

[telectrix]

could it be the interruption of the L by the MCB caused a momentary imbalance sufficient to trip the RCD? a feww mA of N-E?

 

[Darkwood]

No, not my thinking otherwise any S/C on an unrelated circuit on the same phase would have nuisance tripping of RCD's as a big problem throughout the industry, if the L drop voltage it would reflect exactly the same on the N keeping the balance.

Key point I wrote is the motor needs to be started when the Lathe is in neutral but on this occasion it was in gear so DOL on a heavy load.

The motor is about 7.5kw if you need to know.


PS ... heading out for a beverage of the lager kind so you'll have to play pong amongst yourselves, Ill give my thoughts when im back ............Cheers!

 

[ImpededLoop]

My guess would be electromagnetic interference from the mcb causing the adjacent rcd to operate.

Will be interested to hear the answer

 

[telectrix]

magnetic effects on RCD coil due to the high start-up current?

 

[Darkwood]

yep.... magnetic saturation of the rcd balanced circuit due to an effectively slowed down short circuit from the motor stalling on load DOL.... it can happen with a normal S/C on adjacent mcbs but rarely, I repeated it twice and it tripped once.

 

[ImpededLoop]

Thanks for sharing - one to store away in the memory bank!

 

[Knobhead]

Its things like this that underlines the need for electrical theory to be taught. If you know the basics most problems can be resolved by applying a bit of logic.

A lot of electrical theory is basic common sense with a few numbers added. My time at engineering college was nearly all theory. The only time we got to play with any wires was in either the electronics or power laboratories. My son went to the same college. Not even the lecturers were allowed in the power laboratory, far too dangerous according to the HSE. It didn’t seem to matter if we blew ourselves up. The main switchboard in the laboratory had a 400A incomer, plenty of power to fry the odd careless student.

What it did teach us was how to think a problem through. Something I feel is lacking nowadays.

 

[Darkwood]

Tony you nailed it on the head ...we have a no thinking generation coming through - not their fault but its so evident whenever I talk to them, its so basic they are clueless, met the guy who used to work for this firm in CEF ... he tried backtracking why he hadn't done this or that etc.. found the busbar chamber had links in the fuse carrier, he said the owner was been funny with me so didn't fit the correct fuses ...gobsmacked!

 

[Geordie Spark]

Its things like this that underlines the need for electrical theory to be taught. If you know the basics most problems can be resolved by applying a bit of logic.

A lot of electrical theory is basic common sense with a few numbers added. My time at engineering college was nearly all theory. The only time we got to play with any wires was in either the electronics or power laboratories. My son went to the same college. Not even the lecturers were allowed in the power laboratory, far too dangerous according to the HSE. It didn’t seem to matter if we blew ourselves up. The main switchboard in the laboratory had a 400A incomer, plenty of power to fry the odd careless student.

What it did teach us was how to think a problem through. Something I feel is lacking nowadays.

I asked my No. 1 Son to blow my cars tyres up the other day cos I'm not able to due to being poorly and watched in disbelief as he first struggled to fit the hose onto the tyre valve without first removing the dust cap, then struggle some more to work the foot pump.

Last week he tried to ride a bike for the very first time in his 28 years an immediately fell of and grazed his face on the ground.

Unbelievable truly unbelievable!!

 

[Darkwood]

I asked my No. 1 Son to blow my cars tyres up the other day cos I'm not able to due to being poorly and watched in disbelief as he first struggled to fit the hose onto the tyre valve without first removing the dust cap, then struggle some more to work the foot pump.

Last week he tried to ride a bike for the very first time in his 28 years an immediately fell of and grazed his face on the ground.

Unbelievable truly unbelievable!!

pmsfl if you can work that one out... you'll get the jist though...

 

[Rockingit]

Yeah, but I bet he can text and tweet at record speeds!

 

[Geordie Spark]

Yeah, but I bet he can text and tweet at record speeds!

Oh yes - he can that!!

It's a shame that texting & tweeting are not job requirements or he may have found himself a job and stopped being a bloody scrounger before now!!

 

[Engineer54]

Its things like this that underlines the need for electrical theory to be taught. If you know the basics most problems can be resolved by applying a bit of logic.

A lot of electrical theory is basic common sense with a few numbers added. My time at engineering college was nearly all theory. The only time we got to play with any wires was in either the electronics or power laboratories. My son went to the same college. Not even the lecturers were allowed in the power laboratory, far too dangerous according to the HSE. It didn’t seem to matter if we blew ourselves up. The main switchboard in the laboratory had a 400A incomer, plenty of power to fry the odd careless student.

What it did teach us was how to think a problem through. Something I feel is lacking nowadays.

I've been saying that all along on this forum, standards have dropped like a bloody stone since i did my training, but it goes right across the education board as a whole. That increasingly goes for the standards obtained in trade qualifications too. I'd like to see how a newly qualified electrical student would get on with the C&G papers that we had to pass, and there were no multiple choice questions to be found anywhere in those days, full answers often with diagrams only was the order of the day!! Even the attitude of today's students as a whole leaves a lot to be desired.

Yet there are some here that are convinced and will argue till the cows come home that the C&G papers of today are just as hard as those of yesteryear!! Never in a month of Sunday's is my reply to that kind of wishful thinking!!

 

[tidyboiler]

Last week he tried to ride a bike for the very first time in his 28 years an immediately fell of and grazed his face on the ground.





Unbelievable truly unbelievable!!

Hope he didn't mark the tarmac