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galvoguru

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Hello Chaps (and ladies?!)

I'm an electronics engineer that's been offered a job as an electrical design engineer. I've been to have a look at the job and it doesn't seem too complicated however when I had a look at the CAD system they use to layout the circuits I was instantly lost, all the symbols that are used for contactors, MCB's etc are totally different to the electronics world. I borrowed a set of electrical drawings for a machine so I could study them, some things just don't make any sense to me. The first thing is the Emergency Stop relay unit PNOZ1, I've had a look at the internal diagram for this unit but it doesn't make sense (to me!), I can't seem to identify how the contact coil(s) are driven/connected-up, I can see 'K1' 'K2' and 'K3' boxes and these seem linked to sets of contacts... Another item I can't fully understand is the MCB wiring, sure the 3 phases come in, go through the MCB contacts and into the motor, no problem and logical except further along in the drawing list is a page showing showing ALL the MCB's on the machine connected in series (from a 240V supply) and ending up feeding an i/p to the PLC to indicate ALL MCB's are closed, my problem is that the MCB's don't appear to have an additional set of single pole contacts which can be used to indicate 'MCB is closed'. What am I missing guys???

I would be very, very grateful if someone could give me the benefit of their own knowledge to help me out. I've tons of questions that can only be answered by people in the know. Thanks in advance, John.

 

galvoguru

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Sorry I don't know the name of the CAD system. I'm used to using Autocad for the mechanical work I do, but the system I looked at seemed fairly old, I'll spend some time playing with it to see what I can do... AT this stage I would like info about general aspects of wiring practices for industrial machines (see my earlier questions), I'm going to be thrown in at the deep end and I want to be able to swim rather than sink! :)

 
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ok. From your earlier post, a couple of issues present themselves.

If you are making sure all 3 phases come in; you`re NOT dealing with a 240v supply

As regards the i/p to PLC; if all you are confirming is that a series of mcbs have all closed; the o/p from the final mcb would have changed state from *effectively* 0V to phase voltage. There`s your signal!

Hope that helps. As I said earlier; a photo of the drawing would be advantageous.

KME

 

galvoguru

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Sorry, the 240 supply came from a step-down transformer, the 240V being used to power the PLC's, the primary for the transformer was taken from across 2 phases. Re the mcb's, there are probably 10 mcb's scattered all over the machine, some protecting motors, some protecting sensors etc, my point is that the mcb's on the drawing don't show any additional contacts (as well as the 3 contacts used to switch the main juice), do mcb's have 'spare' contacts that switch when the mcb is switched? These can then be used to feed a signal into the PLC to say 'all mcb's closed' ??

 

davetheglitz

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I'm not sure if this is of any use to you - but I've come from a test equipment design background into the electrical world - and more often than not you have to trust your expertise as an engineer.

I've recently had dealings with a compressor star delta starter and have conversed with experienced electricians. Electricians will refer directly to the terminal numbers (connect on 95 and 96) where you will think of it as a momentary action push button - between 13 and 14 on the contactor (NO relay contacts). Once you have a completely annotated diagram you are talking the same language (or may be)!

I really wonder how many sparkys really understand the autocad diagrams - some of the symbols are really obscure - and I'll bet they never apply to a domestic installation and in an industrial situation they are interpreted by an engineer who is on a completely different wavelength. Above all - don't panic. Electrics is electrics whatever background you come from!

Good luck

 

galvoguru

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Thanks for that, I'm fairly sure (like your saying) that electricians and electronic engineers work on different levels and talk different languages, I need to 'learn electrician' !

Cheers...

 
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