We have been asked by operations to look at streamlining isolation tasks for mechanical and process jobs.
I have suggested having local isolators next to the required drives (belt driven fans / pumps etc), which can be operated by anyone, instead of MCC located isolators or even fuse pulling, done by specifically trained people.
I cannot find anything about best practice or pro/cons on something like this, so I a hoping people here could offer advice?
Seems like a good solution to me, provided the isolator would isolate all potentially hazardous conductors (including any for sensors/monitors etc.), and the MCC or other remote control system won't give you unwanted fault/alarm signals if you do only isolate locally.
I assume the drives are generally in secure locations inaccessible to miscreants who may wish to cause trouble by randomly turning your stuff off.
The local isolators can be lockable and the operatives can all have their own padlocks - which all adds to your safe system of work.
Other thought is to check that there's no dependency between individual motors - e.g. you usually don't want to isolate a motor that's driving a conveyor belt if whatever putting things onto the conveyor is still running (otherwise you end up with a big heap of products falling onto the floor and very upset management). Similarly for less obvious things where the process is more complicated (e.g. pumps or fans and heaters - where things could overheat if the fan/pump is off and the heater still on). Keeping single means of isolating a group of things together can help avoid some of the worst mistakes in that respect.
- Andy.
Or something closer to an E-stop loop with key switches so the whole line can be killed and locked off at any one of a few key points. However there are rules about this - a run of series switches and any old contactor will not quite do for isolation for working on parts that may come live or machinery that may restart in a dangerous way if that fails.
Mike.
An isolator is common practice in Industry including flexible leads, plugs and sockets; fitters can change motors etc. without involving the electrical staff. .
If drive is essential to the operation, a means of preventing switch off by unauthorised persons should be considered but still maintaining safety. (I've had phantom switch offs in the past).
Jaymack:
This is something we now do as standard at the site I work at for exactly the reasons specified. We also have two levels of isolation though, one that any trained technician can do that is appropriate for mechanical non-intrusive work and one that only competent Electricians can complete for Electrical work or intrusive maintenance. This stops people from relying solely on the isolator when working on circuits or with their hands in machinery etc.
We also use auxiliary connections in the local isolators as an interlock with the control circuit to prevent the isolators from being used to start and stop the drive. Some isolators come with early break contacts specifically designed to stop a motor control circuit before the main contacts are opened to prevent the device being used on-load.
Hello,
Considering the motor is connected to a machine, you need to look at EN 60204-1 to ensure you remain compliant with the Machinery Safety Regulations (this standard is approved to meet compliance in the UK and EU). It's bad practice to use an emergency stop pushbutton with a lock to stop a machine for maintenance; a suitably rated disconnecting device must be used for the purpose you describe. To avoid repeating what the standard says, see the first paragraph of section 5.4 in EN 60204-1; you need to refer to other areas of this standard too, but this is to point you in the right direction. What you're suggesting is allowed; however, you will likely need to modify the maintenance procedure for this machine in combination with the ISO 12100 risk assessment carried out by the manufacturer. It's possible to add a hazard if you use an extra disconnecting device, so you need to assess how this will be managed. As the standard will tell you, you will likely need to provide suitable signage.
"5.4 Devices for removal of power for prevention of unexpected start-up
Devices for removal of power for the prevention of unexpected start-up shall be provided where a start-up of the machine or part of the machine can create a hazard (for example during maintenance). Such devices shall be appropriate and convenient for the intended use, be suitably placed, and readily identifiable as to their function and purpose. Where their function and purpose is not otherwise obvious (e.g. by their location) these devices shall be marked to indicate the extent of removal of power. Devices for removal of power for the prevention of unexpected start-up shall be provided where a start-up of the machine or part of the machine can create a hazard (for example during maintenance). Such devices shall be appropriate and convenient for the intended use, be suitably placed, and readily identifiable as to their function and purpose. Where their function and purpose is not otherwise obvious (e.g. by their location) these devices shall be marked to indicate the extent of removal of power."
Please don't accept my information as gospel; I am providing these details to point you in the right direction. If in doubt, please contact a machinery safety company with the correct liability insurance etc. Good luck with your project.
I have used this kind of approach in a manufacturing environment in the past to great effect. I now work in the water industry but when I suggested this approach, our VSD framework supplier was dead set against it because of the potential damage to the VSD if it output is open circuited before the VSD has time to shutdown properly. If any of your drives are supplied by VSD it might be worth checking with VSD supplier first.
I still think it is a good approach to take. Solution to the VSD issue and the heap of products on the floor issue mentioned elsewhere would be to have a procedure where there is a controlled shutdown using a cycle stop pushbutton BEFORE the isolator is switched off. The other thing to consider is the need to ensure that the isolator cannot be switched back on - i.e. a lock out tag out (LOTO) procedure needs to be in place. This then raises the question of key control... it should be one lock one key but in my experience this can get messy if the person with the key isn't available when needed to reinstate the drive. Might need some handover process if person with the key is likely to go off shift.
You might need to consider the possibility that the wrong piece of equipment is isolated (e.g. duty/standby kit - person could be injured by unexpected start up if they have isolated the duty machine but started working on the standby machine). Also need to consider proximity of other pieces of equipment... could the person working on the isolated machinery be injured by another machine... does the work require a zone of equipment to be isolated?
While I am satisfied that a local isolator provides a safe and robust solution for mechanical maintenance, I would be a little concerned if the fitter was required to change a motor. Isolators are not infallible and supply can remain on, say, one phase. While this wouldn't be sufficient to start and run a motor, it would leave a potentially lethal hazard for an unsuspecting fitter disconnecting the motor terminals. In such cases I would recommend that fitter is issued with a two-pole tester and proving unit and is instructed on how to prove dead at the point of work - prove it, use it, prove it. Also this suggests a robust maintenance regime is required to check functionality of the isolator periodically especially in adverse environmental conditions.
Another device to consider as the point of isolation is the Marachel Decontactor. This is a plug and socket device that is rated as an isolator ... don't use a standard industrial plug and socket, it is not the same. I used this device on a mixing vessel so that the vessel inspectors were able to isolate for themselves in full confidence that motor tails were fully disconnected and encased in a clam shell which was securely locked and under their exclusive control.
Thank you for the new name to me I have found the marechal website I presume it is these ones here you used ?
I had not seen those before they seem to be quite new, and usefully unusual in a rating for 1kV AC 1k5 DC. Have you been using them for long ? I'd be interested in any feel you may have for reliability in a real setting - the datasheet seems to indicate a flat pogo-stick contact surface rather than the more usual pin and tube, which would be unusual in anything for much more current than a lamp holder - have I misunderstood or is that the case ?
The profusion of pilot contacts make 'safely turn off the VFD' very practical, if as usual the pilot lines are made/broken while the main poles are already or still connected.
Not needed the earth return for a pilot loop in the manner of the normal BS4343 / IEC 60309 is a significant improvement.
Mike
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