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ARC FLASH IN LOW VOLTAGE MAIN BOARD

Former Community Member
Former Community Member
Hi eveyone ! 


I was wondering if there is a conservative table out there that tells which PPE you should use depending on the voltage, normal current, and shortcircuit perhaps of the place you are working.. 


Specially this is because of a new installation, which will have a main circuit breaker of 2000A, in 400V with a three phase shortcircuit fault of aprox 30KA at the point of the main CB.


When we energise for the first time, what type of PPE should we use ? I have been reading and depends on the cal/m2 but perhaps there was a rapid conservative table or experience of yours to tell what type of PPE we should use 


Also, the main circuit breaker is inside the gabinet, and has no dead front to cover the cooper busbars, they are covered in the front with transparent acrylic (6mm depth), in order to prevent to touch them. So to turn on the MCB we have to do it with the gabinet open
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    I've often thought that something like this would be useful, but as far as I know no pre-calculated tables exist for your situation where a few distances and PSSCs for 400V 3 phase would be quite informative..

    If anyone does know of pre-calculated results, I;d be very interested.  For a one off  it is probably easier to over-dress, but it would be nice to be able to estimate the safety margin.

    Mike.

  • Former Community Member
    0 Former Community Member
    Hi Mapj, thanks for the reply


    What would be ideal PPE for your opinion for this case ??


    Just gloves, helmet ? faceshield ? and what grade of protection ?


  • 0
    Franpereira:

    Hi eveyone ! 


    I was wondering if there is a conservative table out there that tells which PPE you should use depending on the voltage, normal current, and shortcircuit perhaps of the place you are working.. 


    Specially this is because of a new installation, which will have a main circuit breaker of 2000A, in 400V with a three phase shortcircuit fault of aprox 30KA at the point of the main CB.


    When we energise for the first time, what type of PPE should we use ? I have been reading and depends on the cal/m2 but perhaps there was a rapid conservative table or experience of yours to tell what type of PPE we should use 


    Also, the main circuit breaker is inside the gabinet, and has no dead front to cover the cooper busbars, they are covered in the front with transparent acrylic (6mm depth), in order to prevent to touch them. So to turn on the MCB we have to do it with the gabinet open 


    I would test all outgoing circuits carefully before energising the main breaker.


    If possible, I would have all individual loads turned OFF before energising the main breaker, and then turn them on individually.


    Z.


     


  • 0
    I don’t know of any “ready reckoner” tables for this, although there is software that will do the calculation. You will need the fault current of the supply, the clearance time and the distance between the operator and the device for the calculation. Once you have the arc flash energy (usually in cal/cm2, you can start working out your PPE. As a base, it should be all cotton (including undergarments), which you would supplement by adding additional layers to get your rating. 


    We (a DNO) wear arc flash trousers, cotton shirts and cotton undergarments as a minimum, then add to that as necessary. Don’t forget to consider hearing protection too. We have simplified to four ratings where basic (class 1) is as above, to class 4, where you shouldn’t actually be there. The preference these days is remote operation so you can stand in a safe location. 


    Regards,


    Alan. 


    Edit: We have two classes of PPE: one for below 8 cal/cm2, and one for 8 to 25 cal/cm2. For values above 25 cal/cm2, you shouldn’t be there when operations are carried out. Most plant these days is operated either by telecontrol, or temporary lanyards rigged up to the operating handles as a first choice. Obviously in some cases this is not achievable.
  • 0
    You can't base it on the voltage, normal current, SCC, etc. because each installation can be different. The arc flash energy depends on the voltage, arc current and time (duration) of the arc, so normal current is essentially irrelevant, but even voltage and SCC alone don't necessarily give the right answer as the clearance time of the breakers/fuses is important and a current just below the instantaneous tripping time of a breaker can give a higher arc flash energy than the full short circuit current.


    Reading your description of the cabinet, I would not be particularly happy working with that arrangement and my best advice would be if possible arrange for remote switching from the other side of the room (or preferably from a completely different room!)


    Alasdair
  • 0
    Thinking about the OP’s situation again, assuming this is a new installation, the safest method would be to energise it remotely, with all circuit breakers in the closed position, then keep clear of the panel for a period of time. 


    Regards,


    Alan.
  • 0
    I am definitely with Alan on this one. Don't forget that the approved PPE doesn't prevent burns/injury, but rather just ensures you have an injured person rather than a dead one.....


    Alasdair
  • 0
    Calculation of incident energy for a specific configuration is problematic , and there is no EN or BSI standard way of doing it at present, which means officially you have little audit trail to say if visor and cotton overalls are more than enough, or at the other extreme if a double brick wall and a bunker is inadequate (common sense suggest somewhere in between).

    For installations in the US, the IEEE publication 1584 (last revised 2018) attempts to assist, by providing formulae that are empirical fits to a number of published experiments involving instrumented tests of deliberate arcs as various parameters change.

    While you need to pay to read the standard itself there is an Excel spreadsheet calculator that anyone who is happy to provide an Email to open a free account can download.

    Having downloaded the spreadsheet, and (as I have IEEE access through work) also read the standard, I must say that I am almost none the wiser as to what actual figures would be applied  to your situation if you were to be using this standard, though there are a number of points worth noting, arcs in enclosures tend to be reflected, so the energy incidence is higher at the open face, than the same distance from an arc free to expand. The corollary of this of course is that it is much better to be to the side of the enclosure, than in the line of fire. The other odd one is that with some 'current limiting' fuses, the arc energy actually drops with increasing PSSC as the disconnection time is faster which is not intuitive.

    As others have said, the best place to be is not there at all - it seems an odd design where the act of switching on requires the only containment to be fully open - I'd expect some sort of inner shield with a cut out around the breaker.

    M.



  • 0
    Alasdair Anderson:

    I am definitely with Alan on this one. Don't forget that the approved PPE doesn't prevent burns/injury, but rather just ensures you have an injured person rather than a dead one.....


    This reminds me of one of the lessons learned in the Falklands war. At that time, personnel would put their designation, e.g. "CO" in black felt pen on the flash hood. Problem was, when there really was a flash, the black section absorbed sufficient heat to brand the foreheads of those affected.


    Nowadays, the designation is written on the flap of a breast pocket, which overlies the pocket itself, so that there are 4 layers instead of 2.


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