Thread by @jonsherrard

I will sometimes get asked a question this time of year: Why, after an extended outage, does my power come back on but then goes out again after a minute or two?

Cold load pickup or, more appropriately, non-diversified load is the culprit.

Electrical load, normally, is diversified. That means that everyone’s heat and hot water tanks are not all on at the same time. My thermostat might run my heat between 10:10 to 10:20, while your neighbour’s might run from 10:30-10:40.

During a prolonged outage, say more than two hours, impacted homes begin to cool off. Power Line Techs make repairs to the line, change and close a fuse, and, voila, power is restored!

The trouble, though, is now everyone’s heat turns on at the same time. How bad is it? Cold load pickup on restoration can result in more than twice the line amperage from before the outage. So, why is that so bad?

Fuses on radially fed distribution circuits are coordinated. There might be 140A fuses at the main entrance to your subdivision, then 65A fuses downstream from that, then 20A fuses downstream from that, etc.

We chose fuse sizes so that only the next upstream device will operate (open) in the case of a line fault, minimizing the amount of customers affected by an outage.

If we miss-coordinate these fuses then a fault that should only cause an outage on a side street could cause an outage to an entire subdivision, or worse. It’s important that fuses be replaced like-for-like to maintain this coordination.

So, a Power Line Tech re-installs and closes a cutout with a 20A fuse. There might have been only 15A of current flowing through the device before the interruption, but the non-diversified load (cold load pickup) means that current is 30A or more.

This amperage is over the steady load rating of the 20A fuse, and while you may have heard a fuse *pop* like a firearm when it operates under a fault, in this instance the fuse will simply melt off under the overload.

What do PLT’s do? Well, sometimes they can install a higher rated fuse. If this would result in considerable miss-coordination then the PLT will have to manually open downstream devices (sidelines, transformers, etc). We’d request approx half of the downstream load.

Then, usually every 20-30mins, the PLT will close a sideline or transformer, slowly restoring customers so that the problem fuse doesn’t become overloaded and load can begin to diversify again.

This impacts the transmission system as well. Best case scenario - we pick up all substation load when we close a breaker supplying a radial transmission line.

Sometimes, though, we have to get the distribution operator to open several substation feeders to prevent overheating the conductor.

I think it’s important to note: our electrical systems experience peaks during the winter season in New Brunswick, but “coal load pickup” can take place any time of year.

Oops - “cold load pickup” not “coal load pickup”. Someday I’ll write a thread about coal generation and the looming 2030 deadline.