Originally posted by: michaelfivey22
"For a 690V system I would have expected initial direct contact but that would then be blown away by the arc as it is initiated and then sustains itself so the initial short may not be detected post event."
Can you please expand on this - the only part in the network we see damage is in the breaker - we can see no signs of damage i the swithboard bus bard apart from the small damage to the back of the downstream breaker cubicle.
With regard to your question above, air, dependant upon the ambient conditions has a breakdown of around 3kV/mm so on an LV system there isn't the voltage level to breakdown the air and initiate the arc. You would therefore need a direct phase to phase contact to initiate the arc and cause the blast.
When we carry out arc flash tests we actually initiate the arc by linking the electrodes together using wire. Then after taking some big strides the power is switched to the electrodes, current flows and the wire between them melts but as it breaks up enough heat has been generated to ionise the air, cause conduction and the arc builds up until the power is removed.
So all you need to start an arc in the breaker is a small amount of conductive debris that initially creates the short but as with the arc test when the blast is over you cannot detect an actual short in the circuit.
I think you mentioned that there was an undervoltage trip coil within the unit. Off the top of my head trip, coils are generally short term rated (as in seconds) do you have a permanent voltage to the trip coil which has subsequently burnt it out and due to the comapct nature of the ABB Breaker has caused an internal short and hence initiated the arc? I don't know how the protection works or if the breaker has internal aux contacts that break the supply to the trip coil but it may be worth considering if you cannot find any other fault.
I don't suppose you have a spare breaker that you could open up or any pictures of the internals and take a look at its internal construction to give you some ideas.
Also from your description I take it that the down stream breaker where you have indications of arcing is situated away from the failed breaker and any arcing debris could not have caused any issues?
Is there anything common between the 2 incidents during the energisation period either time wise or with regard to certain switching actions on other systems?
With regard to the transformer, the windings will move on energisation, as John has pointed out they are usually braced well but I have seen bracing break or move due to forces from fault currents that have flowed through the transformer.
The final thing I can think of at the moment is was any testing carried out on the new breaker prior to installation / energising. I know it is remote but new apparatus can fail and you could be a victim of 2 independant failures.
The 3 guys are actually doing well - all out of hospital and 2 of the three requiring skin grafts. They have all been very lucky. Worth noting though that during the investigation we realised that they were not wearing the full arc flash suit but only the visor and gloves with "bib" overalls. As you can imagin the buring was on the upper arms.
I am glad they are on the mend, burn injuries are some of the worst type of injuries we can suffer and skin grafts are not easy to undergo - I hope all goes well for them.
Unfortunately you have hit on one of the problems I have with PPE, getting people to wear it. I have done extensive work on arc flash mitigation and engineered things down to people only needing to wear a balaclava, face visor and gloves along with their normal FR overalls. We even have it in small kitbags for them to carry around when they go an do some switching but I still find some not wearing it.
The arc energy levels at LV can actually be higher than that at HV so if your PPE is only rated for the HV it may be insufficient for the LV system anyway.
Hence my preference is to opt for engineering solutions.