I was advised by a US customer that they are considering changing the site transformer due to risk/experience of Arc Flash? The transformer is too large and by chanaging to a smaller unit they will minimize/eliminate this risk?. I was referred to NFPA 70E as a guidance document to support the reasoning for this. (I don't have)
I was also advised by our own organization (EU based) the following.."basically, Arc flashing or as we call it in the UK “flash-overs” occurs if the clearance to earth or clearance between phase have been compromised or the insulation material which maintains the clearances has deteriorated which allows liquid or gas which has the effect of compromising the clearances. We design all units with minimum voltage clearances so we never have issues with flash-overs in normal operation. These clearances differ subject to the input voltages (i.e. a 12.76kV will have different clearances than a 38kV).
As long as a transformer manufacturer follows engineering guidelines on clearances and insulation then we never experience flash-overs".
I would appreciate any guidance here in the US on this issue.