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Why do stepping relays fail on pump control panels?

Every pump engineer up and down the country has replaced a stepping relay in a control panel. It's basically a rite of passage - you're not a pump engineer until you've done at least one stepping relay! Most carry a box of spares in the van, and that little box of stepping relays costs £00s. What's going on here?!

So first up, this is the sort of stepping relay we're talking about:

Some people call them switching relays, some call them flip-flop relays, but whatever it's called it's the part that selects which pump is the duty pump.

The normal configuration is that it changes the duty every time the pumps stop running, and to prevent it switching over (for example, because one pump is out of action) you pull the cable out of A1.

Why do they keep failing?

The fact is, this product was never designed to work this way. It's an 'impulse' relay - it's designed to have a short injection of power to make it change state, and not to have power connected for any length of time. The way they're usually wired in a pump panel they have power for as long as neither pump is running. In some pump stations that can be for a number of days. They get hotter and hotter and eventually the magnetic coil inside melts.

What's the solution?

Obvious thing to say, but use a relay that is rated for a permanent power connection! Our preferred way of doing it is to use a Universal Relay, which has one terminal for a permanent power connection and a separate terminal for the trigger input to make it change duty from one pump to the other. This means that the trigger input isn't directly connected to the magnetic coil and so there's no danger of it over-heating. It can also be done using a latching relay, but this can be quite an expensive way of doing it.

But we know how the old type of stepping relay works!

You'll be pleased to know that the Universal Relay is actually simpler to use...remember the bit about removing the cable in A1 to stop it switching duty? The universal relay has a dial where you can just select pump 1 or pump 2, to permanently select that pump without pulling any wires out. In every other way it works exactly the same as the old type. And on top of all that, one of our technical specialists will come to your offices or a site you're working on to demonstrate how it works - free of charge.

The revolution in stepping relays started here, and you're part of it...


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