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Advice on Replacing a Blown 125A Fuse on a Large Motor, Best Practice?

Rob Phillips

I’d like to ask for opinions on a theoretical scenario to clarify best practice.


Imagine there’s a large motor powering a large water pump on site, supplied via a 3-phase circuit with a 125A 40kA BS88 Gg fuse. The fuse on the red phase has blown.
The person on site holds the 18th Edition qualification but has limited hands-on experience. They have confirmed (under full LOTO conditions) that the motor is freely spinning and the pump hasn’t seized. A subcontractor has advised them to simply isolate the supply, replace the fuse, and re-energise the circuit. However, the site manager (25 years’ experience and 2391 qualified) has stated that the circuit must be tested first to determine the cause of the fault before restoring power.
The subcontractors are insisting it’s the on-site person’s job and are pressuring them to just swap the fuse.

My view:
I would go with the manager’s advice and insist on testing prior to replacing the fuse. Simply swapping the fuse without investigating could mask an underlying fault and lead to further damage or safety risks.

Questions:

What would your opinion be in this situation?
What guidance or regulations would you reference to support your view?

Parents
  • 0

    Depends what you define as tests - a quick flash with the meter to show no winding to case short may be in order, but there is more none instrument info to collect before firing up willy-nilly. 

    Do we have any other additional info -
    has this fuse blown before, 
    was the motor still turning on the other 2 phases, are the other 2 fuses OK ?
    were there any usual circumstances prior to failure, (running dry, running hot, repeated power cuts and restarts in rapid succession  ,,,) what is the plate rating of the pump, has any other kit on that phase been misbehaving?
    Smells of burning, water in places it should not be are all non electrical clues to be considered.
    what is the general age and condition? (that applies to the motor and starter, the available testgear and in some cases the people...)



    One has to see the subbie has least to lose if it goes wrong, just a chance of more work later and the local manager perhaps the most in terms of career limiting avoidable additional  damage to the plant. This gives more weight to the opinion of the latter.

    In the end the go / no go decision of 'am I competent for this task ?' is a tricky one - and best assessed by those on the ground - the chap on-site has to be happy with what he is about to do .

    Fuses do fail from old age occasionally, but usually there is a reason.  There is presumably not a lot of scope or advantage to advanced testing - and beyond insulation test and quick look in the starter for a jammed contractor etc- assuming  its the kind with one, soon one gets to a point of  'try another fuse and see ' with one hand hovering over the 'stop' button and everyone aware it may be a dramatic moment.

    Mike.


  • 0 in reply to mapj1
    mapj1 said:
    Fuses do fail from old age occasionally, but usually there is a reason.  There is presumably not a lot of scope or advantage to advanced testing - and beyond insulation test and quick look in the starter for a jammed contractor etc- assuming  its the kind with one, soon one gets to a point of  'try another fuse and see ' with one hand hovering over the 'stop' button and everyone aware it may be a dramatic moment.

    My father drummed into me from an early age that one should try to establish why a fuse blew rather than just replacing it.

    From the sound of it, there has already been a thorough inspection.

    If the motor was overloaded, perhaps all three fuses would have blown? A loose cable might make contact with an earthed part of the equipment, so have a look in any junction boxes, etc.

    As Mike says, an IR test might reveal a L-CPC fault (I am assuming no neutral). It may be worth checking the resistance of all three windings: if one has overheated and melted the insulation, there could be an internal short, but not necessarily to ground. External signs of this might not be obvious.

    As for guidance, etc., clearly there needs to be a safe system of working under Health and Safety at Work Act 1974 and its subordinate legislation such as EAWR 1989. I suspect that diagnosis is largely a matter of professional knowledge and logic.

  • 0 in reply to Chris Pearson

    I might add that if you replace the fuse and it goes bang again, you certainly cannot repeat.

    If you replace the fuse and the motor runs normally, will you be confident that it will not blow again tomorrow or next week? If the machine fails again and somebody is harmed, can you really say that it was safe to put the motor back into service?

Reply
  • 0 in reply to Chris Pearson

    I might add that if you replace the fuse and it goes bang again, you certainly cannot repeat.

    If you replace the fuse and the motor runs normally, will you be confident that it will not blow again tomorrow or next week? If the machine fails again and somebody is harmed, can you really say that it was safe to put the motor back into service?

Children
  • 0 in reply to Chris Pearson

    Also, what tests should be performed once the fuse has been replaced and the motor restarted? 

    Will someone monitor the on-load running currents etc. ? Also leakage paths (diverted/out of balance currents, etc.)

    Just a thought Shrug 

  • 0 in reply to Philip Oakley

    it depends what it is, some kit is just designed to be run until it fails and then replaced.

    It may be more or less critical however, and if it was say water purification for a hospital, I'd expect far more care than if it was one of a half a dozen identical units at a sewage works.

    There  may be a VSD that provides some diagnostics. At the very least it should be checked after its been running for a bit for signs of overheating.

    I'd be surprised if something that size was direct on-line.

    Mike,

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