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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?

  • 0 in reply to mapj1

    Hi Mike, 

    The tests requested was a IR, test and ohms test on all winding along with inspecting load balancing and kit in the control panel like thermal overload

  • 0 in reply to Chris Pearson

    Hi Chris, tests was requested as I forgot to mention the red phase had blown already some time back in the last two years. 

  • 0 in reply to Rob Phillips

    A couple of further thoughts: (1) is the motor running continuously or intermittently, e.g. to pump away flood water? If intermittently, the interval between the fuses blowing may be quite short.

    (2) Is the loading on the motor marginal, so a small overload of long duration? Bear in mind that the load could have increased since installation 33 years ago.

  • 0 in reply to Chris Pearson

    Pulsed operation like a pump with  (say) a dicky float switch or a loose connection could lead to multiple starts in a short time, while it is probably only intended to do a few per hour and cool off between.  There are a lot of things to understand and factor in a real system. Local knowledge, which we do not have, is key. It is why I dont like  a rigid  'code book' approach to fault finding - its a good start, not the last word. 

    I mean for all we know, it failed in a thunderstorm ! 
    Mike

  • 0

    A good place to start is to work out what caused the fuse to fail open.  

    Is there or was there a mechanical fault? eg did the motor seize or did the pump run dry?

    What else was happening onsite at the time? were large power load switching on and off causing surge/spike?

    What was the weather condition.  Heavy rain causing damp or lightning strike casing a transient?

    What does the maintenance chart for that device/asset have in recent history?

    The kind of questions above are normally best served/answered by someone onsite or with local site knowledge.

    Hypothetically what would happen if the fuse was replaced without any tests what so ever the the fuse blew again?  Could it damage/destroy the asset in question?

  • 0

    I''d go with fault finding first, IR the wiring installation and understand where the fault came from, could be different scenarios of blowing that fuse , inrush current, overload ,short circuit , unbalanced currents etc. Replacing a blown up fuse without establishing the cause is a quick fix to get out of site and make temp solutions. As other mentioned , it might happens again in weeks time and start over again. 

  • 0
    Best practice is to investigate before replacing the fuse. A blown 125A BS88 fuse indicates a fault—possibly short-circuit, insulation failure, or overload. BS 7671 Reg. 131.3 and HSG85 require verifying circuit integrity and fault cause before re-energising. Simply swapping the fuse risks recurrence, equipment damage, or arc-flash hazards.

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