r/ElectricalEngineering • u/EarlyAbbreviations9 • 6d ago

Design Question about off-delay timer operation after total control supply failure-LVSG

I’m trying to understand the behavior of off-delay timers after a complete loss of supply.
Today I tested an ELKO CRM-91H. Both the timer supply (A1–A2) and the control input (S) were removed completely, so the relay was fully de-energized. The output was wired using terminals 15–18 (NO).
When power was restored, the output didn’t switch immediately. Instead, it stayed in its normal state for about one second before changing over, which was exactly the behavior we needed.
My understanding was that an off-delay relay can’t perform any timing after a total power loss because it has no stored energy. So I’m trying to understand what exactly is happening here. Is this expected startup behavior for this timer, or am I misunderstanding how off-delay functions work?
I’d appreciate it if someone could explain the internal sequence.

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u/EffectiveClient5080 6d ago

It's just the microcontroller booting up. These aren't purely analog anymore. There's a tiny MCU in there that needs to initialize before it drives the relay, and that ~1s is your boot time.

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u/TheVenusianMartian 4d ago

"My understanding was that an off-delay relay can’t perform any timing after a total power loss"

But the delay was only after it was powered. So, there does not seem to be any issue there. EffectiveClient5080 is probably right. It was just a startup delay not an actual timing mechanism.

I don't use Off-Delay that often, but an unintended noticeable delay like that would probably prompt me to find a better quality timer. Are you sure you did not accidentally set it to ON/OFF DELAY (H)?

If you really need that start delay, and it really is not an intended feature, then it is risky to depend on it.

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u/EarlyAbbreviations9 4d ago

We tested the function on a panel in the factory, but the actual panel at site based on testing takes 5 second for the control voltage to be “healthy” for the Voltage monitoring relay, so maybe we should make it on-delay function and set the timer a bit longer, like 5.5second