Glad I didn't switch THIS on.

A mate of mine used to work on large (>1000A/phase) ventilation motors during a local motorway construction project. They were having issues water leaks in a particular segment of the tunnel. Some poor bastard went in to reset a 50kVA supply breaker after a switch room had flooded and the bloody thing just vaporised itself, along with two 10mm thick copper bussbars and most of the other equipment in the cabinet.... the supply authority hadn't isolated the supply at the submain transformer, for some reason, but the foreman had said it was already made safe so he didn't bring his test lamps. The guy survived, barely... he lost sight in one eye due to the flash and had some very nasty burns. Several companies copped some pretty severe fines after the incident apparently. This kind of equipment is manufactured to a very high standard, but it can never withstand the combined forces of human error and physics. Not To Be Messed With.

Chris Talbot

Failed when switched on. Reminds me of a time long ago...walked into a building at work the morning after fierce electrical storms the night before. No power. Went to the panel, and the 480 VAC 3-phase main was tripped. Okay, turned on the main, all was well. Saw that the breaker for the surge arrestor was tripped. Reading the instructions on the surge arrestor - "if tripped, reset breaker." So we did that. As soon as that breaker made contact, KA-BOOM! Fire shot out from the sides of the panel! Turned it back off...and changed our undergarments... The electricians noted that some dumbass had jammed a three-phase delta arrestor into a Wye circuit when the building was built...

Benjamin Hall

I really enjoyed the breakdown of this unit (pardon the pun)

Michael Thompson

I like JW's dry sense of humour.

Neutral in. Here's a wiring diagram: <a href="https://i.imgur.com/cdENKgF.jpg" rel="nofollow noopener" target="_blank">https://i.imgur.com/cdENKgF.jpg</a> (the line in at the top is a busbar).

What’s the blue cable fer?

Those lockout devices are neat, but a full kit for all the different breaker types isn't cheap, plus the need to keep suitable padlocks and all the keys for them can make them a little impractical. We generally still rely on the good old twist of wire through the holes with a lockout tag attached. The sight of yellow &amp; black warning tags (and the very stiff penalties attached to unauthorised removal of same) are generally considered to be a sufficient deterrent for anyone tempted to mess around with the board.

Chris Talbot

It's a long time since I've drilled myself, but it can still happen.

Big Clive

I learn about electrical safety from these videos. Power tool safety? Not so much. :P

So... you're suggesting that an industrial AC induction motor won't have a high inrush current, and I can just specify the supply/breaker capacity and the cable to suit the nominal steady-state load without causing any issues...? Hmmmm...... I'm sure what you meant to say is that both highly capacitive loads AND highly inductive loads can cause large inrush currents at switch-on. ;) Welded contacts might have been a cause of failure, but given that the internal switch mechanisms of these MCBs/RCBOs are designed specifically to prevent that from happening, I'd say it's unlikely. Although I suppose if the load was pushing the limit of the breaker (but not quite high enough to cause a switch-on trip) it might be possible for the contacts to briefly arc over and weld together at switch-on, which would cause a catastrophic failure later when the device is tripped.

Chris Talbot

High inrush is caused by capacitive loads not inductive :P Old school linear power supplies had this problem. Good video. Possible cause may have been due to the contacts welding and the protect circuit trying to disconnect a welded contact.

BB

I think it failed when switched on.

Big Clive

Lightning?

B3