solenoid valve & relay inrush currents

  • Thread starter Sumeet Chimalkar
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Sumeet Chimalkar

Does anybody know of any guidelines for sizing fuses and cables for using with relays and solenoid valves. Is it necessary to use the inrush current for sizing or the steady state

Thanks in advance

I've always used the inrush current, but I am not an electrical engineer, so YMMV.

Walt Boyes

Walt Boyes -- Director of New Business Development
Branom Instrument Co.-- P. O. Box 80307-- 5500 4th Ave. So.
Seattle, WA 98108-0307
Phone: 1-206-762-6050 ext. 310 -- Fax: 1-206-767-5669 --
mailto:[email protected]

Johan Bengtsson P&L Automatik AB

What normally makes a relay break is the current present when the contacts open. This is also why there are different currents for DC, AC and AC with inductive load.

I have seen a relay, rated 400A DC. The peak current was about 1100A for like 30s and then it slowly dropped down to about 300A before the relay contacts opened. This worked fine as long as the relay didn't open too early (when that
happened there was nothing left of the relay).
I don't say that this was a good design but the relay didn't take any serious damage from it under normal circumstances.

I think the answer is found by asking yourself those questions:
1. What causes the inrunsh, and most important, how long time does it take before the current is at normal values.
2. How likely is it that the relay contacts will open before the current reach normal values.
3. The cost of each alternative: Replace a broken relay (including downtime and so on) compared to a more expensive relay. With regard to the probability in (2). Think it thru, is it something very important: just use the inrush current.

If the inrush current is really high (a lot of times bigger than the normal current) and last for some seconds or more you might have to consider the generated heat from the current
too inside the relay, but otherwise that isn't an issue.

The above is valid for mecanical relays, for solid state relays you probably have to consider the heat generated by the inrush in a much hinger degree.

AC is generally kinder to the relay since the current crosses zero all by itself periodically.
Having an inductive load is on the other hand worse for the relay (both for AC and DC circuits) since the inductance will force the current to continue after the contacts open.

/Johan Bengtsson

P&L, the Academy of Automation
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
That remembers me a fellow worker of mine. Before arriving in America, He was working in RATP (Régie Autonome des Transports Parisiens). He was telling me they were cycling relays and contactors one million times before putting in service. I have no proof.