I am working on a module that allows a small internal-combustion engine to be shut down remotely. These small gasoline-powered motors are of various makes and models and all have magneto ignition systems.
The ignition systems can have either electronic or Kettering (points and condensor) ignition.
The most-common method of stopping these motors is to ground a node within the ignition system. For Kettering systems, one simply grounds the points. For electronic ignition systems, a node is provided that has the same functionality.
My customer wants me to provide SPDT relay contacts. I prefer to use Telecom-grade relays for this application because there is the possibility that the relays may need to switch low-current / low voltage-level signals.
Problem is that the maximum voltage present at the "kill engine" node is a spike that approaches 400V peak. All of the Telecom relays that are readily available to me have maximum voltage rating of 250V.
However, these relays are all DPDT. Theoretically, I should be able to connect the relay poles in series and get double the maximum voltage rating. Theoretically.
My question is: is connecting relay poles in series to get higher maximum voltage a reliable solution?
[Edit]
It's been pointed out that "All the pain is taken by the last to close and first to open." I agree completely. But please follow my logic here and see if the following makes any sense at all.
First: the relay operates only to either allow the engine to run or force it to stop.
When the relay operates, the poles do NOT operate simultaneously. There is a lag of some hundreds of microseconds between when each contact changes state.
But: does that matter? The only time this is a consideration is when the relay is changing state. When the engine is supposed to be running, you have two sets of contacts in series with, theoretically, double the rating of each individual contact.
When the relay goes to the state that will allow the engine to start, the engine is currently stopped. There is currently NO ignition present, and therefore no voltage on the contacts.
When the relay goes to the state that shuts the engine down, one contact closes first, then the other. It is possible that the contact that closes last has a single spike that could breakdown the gap. Before the next spike can occur, the other contact has closed and there are no more spikes.
The total energy contained in a single spike is really quite tiny. I don't have hard numbers right now but I will get them when I start testing with real engines. But I don't believe there is enough energy in the spike to damage the contact.
And, I do suppose that I can monitor the voltage across the open contacts (both open) and allow the relay to change state only immediately after the spike has decayed to a low value. That completely gets rid of the voltage across a single contact.
But my original question stands: can I count on the breakdown voltage to be double that of a single contact if two contacts are wired in series and not changing state?
Best Answer
Quoted relay voltages are usually set by the switch-opening process. Not by open-switch standoff voltage.
In other words, your 250V relay can tolerate the severe arcing which occurs whenever a 250V circuit is suddenly opened. The same relay probably can handle far higher voltages before it would arc across, as long as the contacts weren't opening during the moment the 400V spike occurs. (And even then, it might work OK)