regulation 1

[bigheadted]

I am working at a plant that after many years of denial is being dragged kicking and screaming into our current heath and safety regime
my problem is that the health and safety department are insisting that full isolations be carried out on our machines before the cleaners, who uptill now have relied on estops can work inside the machine. A resonable demand but it causes me problems as the dc drives that contrl these machines dont like being powerd off. pleanty of past experiences tell me that more than occasionally they go off and then stay off.
My intended solution is to interupt the field and armature connections to the motor with a local locable isolator that can be locked off by our electricians.
i have some questions about this approach.
i intend to put the breaker inside an automatically locking box that can be unlocked by the combination of the main contactor open signal on the drive and some sort of access button being pressed i intend the opening of the box to inerupt the stop circuit of the drive to make double sure the drive is not powerd up when the isolator is switched.
some of these signals will be plc controlled, some via a comms link. is this acceptable?
how do i go about rating an isolator for dc are there any funndemental considerations?
Is this really necessary do the regs really request an isolation. how can i find out more about the regs for rotating machinery isolation?

 

[itsmoked]

Just seems to me that if people are getting into machines the power should be shut of.. And locked off.. and the only key in the cleaner's pocket.

Also you should be able to turn off the equipment and turn it back on without it failing. If it does fail fine! Improve it when you repair it so it meets reasonable manufacturing reliability standards.

I would not go to some non-conventional technique for safety!! I don not understand your safety scheme from a reading of your post. This means it is too complicated!!! and will be misoperated at some point. Can someone be injured during a misoperation? (Sounds like it to me.)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

When talking about regulations it helps enormously if you mention which country you're in.

Are the isolators ever going to be used for load making or breaking? This introduces some very different conditions compared to off load operation.

I would be looking for a recognised isolator / disconnector / breaker with a recognised locking device. I would think very carefully before engineering my own safety system, and given the questions you're asking I'm not convinced that you have the experience to design something which will guarantee to provide 'safety from the system', to quote a phrase beloved of the UK's HSE. That's important if it ever went to legislation because you overlooked something which directly or indirectly lead to an accident. Your defence is much stronger if you have used a recognised product in the way it is intended to be used and within its designed rating.

If you're in North America, Cutler-Hammer's range of MCCB's are available without a tripping unit suitable for 250V DC service, from a few tens of amps up to many hundreds, which are capable of accepting a locking device. If you don't need load switching capability then these are probably overkill.

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Sometimes I only open my mouth to swap feet...

 

[bigheadted]

i will simplify my explination:
i intend to interupt the output of the drive before it gets to the motor therefore isolating the motor.lockable. i intend to provide electrical interlocks to prevent the drive being isolated at his point whilst power can be supplied to the motor ie to stop some idiot switching it off whilst its running.
is it resonable to monitor the fact that its running over a comms link.

 

[davidbeach]

You can get (at least in the ANSI world) disconnects (both fuseable and nonfuseable) with aux contacts, including fast opening aux contacts. Using a disconnect with fast opening aux contacts, and the aux contacts wired to the drive controls, you can have the drive shut down before the power blades of the disconnect open. As long as the disconnect is open, the drive will not try to energize the conductors to the disconnect. That way you have a lockable device that opens the power conductors and simultaneously turns off the drive controls. No need to invent anything.

 

[waross]

I don't like to see any switching in a field circuit. Some day it will fail open with disastrous results. I agree with Itsmoked on repairing or upgrading the drives so that they may be turned off without failing.
As for using the comm link. You know your own comm system better than we do, and the fact that your asking the question makes me doubt that it is a good idea.
respectfully

 

[jraef]

Just so you know, there should not be ANY problem turning power on and off to the drive, if it is not running! The damage comes from opening a circuit under load. If you have given the machine / drive a stop command first, [/]then[/i] open the safety disconnect, you should be able to do that as many times as you like without harm.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[DickDV]

jraef, the one big exception to your statement about opening a disconnect on a non-running motor and that is the shunt field circuit.

On even rather small wound field DC motors, opening the field circuit with a contactor results in a horrendous bolt of lightning due to collapsing flux in the motor field.

Any de-energizing of the field circuit should include suppression devices to manage the kickback energy.

 

[IRstuff]

[red]I think this is a BAD IDEA![/red]

If your modifications fail to do their job, you may KILL someone. I can't see how your management would even begin to contemplate the possibility of risking someones life during a trivial cleaning operation.

The lawyers would have field day and make the cost of fixing the machine to operate correctly seem like peanuts.

TTFN

 

[bigheadted]

respectfuly; i am stuch within the system i have got i agree the drives should power down ok, the fact is they dont and despite sending the cards back to manufacturers for repair they tend to do the same again at some later date?
How can these modifications fail. maybe i should have mentioned this is a separately exited field dc motor, the isolator will interupt all 4 cables to it so that it can recive no power from the drive. the interlocks i propose are to protect the machine from being turned off with power on for the resons suggested by jraef, to protect the machine for the not the cleaner.
i intend to make sure the e stp citcuit is open, by hard wiring and the field/ main contector is open using a signal from the drive ofver a comms link. two layers of checks to prevent power being supplied to the field or armature before access to the isolator is allowed. once isolated the aux contacts of the isolator would also interupt the stop circuit to prevent the drive being powerd up
There is a field fail routine within the drive logic to detect field current before closing the main contactor also