Circuit breaker: Derating is not downgrading

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What is circuit breaker derating?

To some people, “derating” sounds like “downgrading”. Derating is in fact the rated current guarantee for a device installed into an assembly. As if a circuit breaker, for example, is somehow substandard if it operates below its nominal current when actually mounted in a switchboard. Actually, derating makes devices safer and more reliable. Here’s why.

Take a device – a circuit breaker, for example. It complies with standard IEC 60947 which spells out its performance requirements. So a manufacturer will test its breaker against the benchmark of IEC 60947 – i.e. in a lab, upright, standalone, at 40°C ambient temperature, and with cables 2 meters long. But out there in the real world, things are different.

What’s different about the real world, and why is derating important?

Pretty much everything is different in the real world. The breaker is mounted sideways, connected to other devices, and has a 4-meter cable run, its. And internal ambient temperatures in LV switchboard are higher because of heat dissipation from other devices. In an assembly wire lengths and cross section can be different, which affects conduction. Similarly, convection in an enclosed assembly is lower than in standalone test conditions. Radiation, too, is reduced, though not by so much.

We now have to think in terms of IEC standard 61439, which spells out performances for assembled devices. And that’s where derating comes in. In specific assembly operating conditions, devices may are assigned lower currents.

Who tests what and how?

At this point I’d like to distinguish between manufacturers who make integrated assemblies – i.e. breaking devices, busbars, installation systems, enclosures, etc. – and vendors of, for example, installation systems, who don’t sell products. Or who sell busbars or cabinets, but not installation systems.

Whole-system manufacturers test their devices both individually and as part of an assembly. If such a company – usually one of the big ones – produces a busbar, for example, it will test every device (fuse, CB, thermal relay switch, variable speed drive, etc.) connected to the busbar. It will assign a rated current to the busbar and everything connected to it.

Take a circuit breaker for which IEC 60947 stipulates a current of 250A at 40°C. Because the internal ambient temperature of the assembly is 50°C, or more, the whole-system manufacturer need to take into account this using constrain and will derate the current by say, 10%, to 225A. In fact, the manufacturer’s technical guide contains tables that specify the derated currents for all assembled devices. It’s the reliability guarantee and safety to the customer. In a word for everything that’s installable in a switchboard. Sound complicated? Painstaking? I suppose it is. But derating devices ensures safety and reliability that devices tested against IEC 60947 don’t.

Pity the poor panel builders

I mean, how can a busbar vendor who doesn’t make other products give current ratings for a whole system? A vendor who says “Oh, I tested my busbar at 40°C, so it should be okay if temperatures don’t exceed 40°” is jeopardizing the whole assembly.

Nothing is more dangerous than presuming an individually tested busbar will work okay in an assembly with breaker made by another company. Each device may be fine. The result will not be.

Pity the poor panelbuilders. If they buy different components from different vendors who have not-assembly tested them, what do they get? A mix of products that meet components standards individually but not collectively.

With their exhaustive assembly-tested derating tables, the big companies necessarily tell the whole truth and nothing but the truth. Product and installed system vendors – unwittingly – don’t tell the whole truth.