UPS Tech blew 40 500A fuses after cycling battery cabinet breakers for a powered down unit - Does anyone have insight in to how this would happen?

[Mamoo]

I do building maintenance for a data center and at one of our sites down the street we had a testing company come out to do some maintenance on the breakers for each battery cabinet in a UPS. From what I understand after he was finished he wanted to cycle the breakers to make sure the hammers were operating correctly.

We told him to hold on so we could make sure it was kosher to do this. UPS was already powered down, input breaker from switchboard open etc.

He went ahead and bypassed the shunt trip so that he could operate the breaker and after cycling each breaker with the UPS off he blew every fuse in each battery cabinet.

I just want to know how this could happen. Because if your UPS is off and not feeding a load, it's not like you're slamming 1000-2000A on the unit. At the same time you're also closing a DC circuit with charged batteries that feeds your DC bus. Would this still have an effect even?

 

[zbang]

Making a few assumptions here, if the batteries are being connected into a completely discharged capacitor bank, the inrush could blow a fuse. OTOH once a couple blew, you'd kind of expect the tech to stop testing and figure out what's going on. Blowing forty fuses? That negligent and the testing company owes the facility a lot for replacing them.

(As with most things like this, a diagram of connections can be a great help.)

 

[Mamoo]

Yeah the tech said he didn't hear anything, I haven't been next to a 500A fuse when it blows (let alone 4 in each cabinet) but I can't imagine it doesn't make any sound. Let me see if I can find the drawings, it's a Mits UPS but they have 300/500/750 kva models and I'm not sure which ones we have.

 

[Mamoo]

https://www.mitsubishicritical.com/...750kva-ups-direct-cable-submittal-package.pdf
Found the manual, going to go out on a limb and assume it's their 750kva model. I have a few years experience in the trade before getting out of construction, so I have a tertiary understanding of this but still very green in a lot of the more technical aspects of stuff like this.

 

[Jraef]

Making a few assumptions here, if the batteries are being connected into a completely discharged capacitor bank, the inrush could blow a fuse. OTOH once a couple blew, you'd kind of expect the tech to stop testing and figure out what's going on. Blowing forty fuses? That negligent and the testing company owes the facility a lot for replacing them.

(As with most things like this, a diagram of connections can be a great help.)

That's likely exactly what happened. By bypassing the system, he also bypassed the "pre-charge" circuit that prevents the capacitor charging current from spiking and damaging everything. Capacitors charge instantly, pulling ALL available current in the system in doing so; they are like a bolted short circuit for that first instant. When you first energize them, you have to control that current, either by ramping the voltage into the circuit or by putting a current limiting resistor in series for a few seconds or so, then shorting across the resistor once the caps are fully charged. The shut trip that he bypassed may have been there to PREVENT this from happening...

 

[Mamoo]

That's likely exactly what happened. By bypassing the system, he also bypassed the "pre-charge" circuit that prevents the capacitor charging current from spiking and damaging everything. Capacitors charge instantly, pulling ALL available current in the system in doing so; they are like a bolted short circuit for that first instant. When you first energize them, you have to control that current, either by ramping the voltage into the circuit or by putting a current limiting resistor in series for a few seconds or so, then shorting across the resistor once the caps are fully charged. The shut trip that he bypassed may have been there to PREVENT this from happening...

Very interesting and that makes a lot of sense. Thank you both for the information