There continues to be quite a bit of discussion about an arc flash creating enough pressure to blow equipment doors open. I have done this in the lab on many occasions and have seen it in forensic analysis (as have a few others). However, this week, we’ll look at actual experiences.

Have you/company/clients ever experienced an arc flash where equipment doors/covers blew open?
Yes
No

If yes, what was the cause of the incident?

The link below is a video that I shot in the lab of a 400A bus plug with it's doors closed being blown open by an arc flash at 600 volts with 23,000 amps. I posted it in a response last week so it was worth reposting for this week's question.

Video-Doors Blowing Open

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I have seen this twice an in both occations I believe that the transformer secondary cable were overheating do to a fault in underground conduits which caused the cables to emit vapors which when the fault occured ignited these gasses and caused piling of the gases through the conduit until the mixture reached both ends and ignited. In once instance blowing the pad mount transformer enclosure about 10'. The other occation blew a NEMA 4 enclosure door open by bending all of the stainless steel clamps open. Both occations also blew gear doos off.

We recently were asked to investigate an incident where the doors were blown off a medium voltage MCC supplying a 1250HP motor. An overload at the motor initiated the MCC fuses to clear the overload. Apparently, there was some weakness in one of the fuses causing it to outgas into busses which were not protected by any kind of shield thus triggering an arc which proceeded to blow the doors off. The fault current was estimated to be in the vicinity of 25.5 kA. Fortunately, no one was near the equipment but the incident was captured on the security camera.

An outdoor 15 kV vacuum in air circuit breaker was found with the low voltage cabinet doors open on both sides. Investigation found the heaters in the high voltage compartments were connected to thermostats that were left switched off at the factory. Lack of heat and poor venting caused condensation to form on the interior parts of the bushings until they flashed over. The flash vented into the low voltage compartment, blowing the doors. The high voltage doors are bolted, and were not affected.

I didn't see it, but I showed up for the aftermath. A client was racking in a full height breaker with the door closed. This was Federal Pacific Gear (a number of years ago). No one is sure exactly what happened, but the breaker arced during racing and blew the door open.

Jim do you have any other data around the test in the video? What was the clearing time and the incident energy? Also would be curious to know the AIC rating of that bus plug.

Doors blown open or off? I've seen doors blown open (latch ripped apart) in some 4160 V gear but blown off in 480 V MCC's is pretty common. Multiple accounts and forensic investigations of people trying to plug buckets onto energized 600 V class MCC's tend to have similar results...the arc initiates at the bucket stabs, travels down the vertical bus to the bottom, then blows the small door off the bottom of the MCC section.

Plus lots of cases where motor peckerheads blew a door off or sometimes the bolts ripped out and the whole peckerhead cut loose. There is some debate though about this one because it is also known that certain greases when used in motor bearings can catch fire and blow the peckerhead off. I used to believe that this was every case except that in one particular case having worked at this point for both sides (customer and vendor) I am very certain that the correct grease was used (Polyrex EM).

Plus...lets face it a typical size 3 starter is going to have roughly 250 square inches of metal door (16" x 16" allowing for a 4" vertical wireway). Measured pressures in the lab vary all over the place but the highest values measured for very well sealed enclosures are around 8-10 PSI so we can expect easily up to 2500 lbs. of pressure on two 1/8" pins on the hinge side and a thin say 16 gauge sheet metal latch on the other side. No way it's going to stay on.

I have seen the after effects of 2 incidents,

One on Medium Voltage Motor Starter fed from a 10 MVA transformer. Moisture ingress along the incoming bus duct made it to the starter and eventually caused a phase to phase fault. This was 70s vintage Westinghouse gear and withstood the blast with the doors bent outwards but the door clasps held.

I have also seen an older Allen Bradley MCC blow the door open on the bottom section when the fault traveled along the bus.

Yes, incidents involving doors or covers being forced open during an arc flash are definitely possible, especially when pressure builds rapidly inside enclosed gear. In several investigations, the main issue was not only the electrical fault itself but also trapped gases, deteriorated insulation, or poor ventilation inside ageing equipment. Once the arc developed, the pressure wave had nowhere to escape and the weakest point became the doors or panel covers.

One case discussed during a refurbishment project involved low-voltage switchgear where contaminated busbars and loose terminations created sustained arcing. The blast pressure distorted the hinges and partially opened the front section of the enclosure even though the fault duration was relatively short. Situations such as these are a reminder that mechanical design and enclosure integrity matter just as much as protection settings.

A lot of firms involved in commercial and technical space planning, including Molecule Designs in the UK, increasingly pay attention to how electrical rooms are integrated into buildings, particularly regarding safe access, ventilation, and maintenance clearance. Good layout decisions can make inspection and preventative maintenance much easier, which ultimately reduces the likelihood of these faults escalating into dangerous pressure events.

The video is a strong demonstration of how quickly an arc event can turn into a mechanical explosion rather than just an electrical fault.