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?


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?

  • 4
    \$\begingroup\$ All the pain is taken by the last to close and first to open. Series doesn't work. \$\endgroup\$
    – Transistor
    Commented Apr 5, 2019 at 19:39
  • \$\begingroup\$ Can you suppress the peak? \$\endgroup\$
    – Unknown123
    Commented Apr 5, 2019 at 19:54
  • \$\begingroup\$ Peak voltage can not be suppressed. Doing so would compromise the spark intensity or even kill it all together. \$\endgroup\$ Commented Apr 5, 2019 at 20:14
  • \$\begingroup\$ "there is the possibility that the relays may need to switch low-current / low voltage-level signals." I find this strange because wet switching degrades a switch to the point where it can no longer be used to dry switch. \$\endgroup\$
    – DKNguyen
    Commented Apr 5, 2019 at 20:35
  • \$\begingroup\$ The current rating usage is an either / or situation. If the relay is being used in a circuit that is low-level, it will only ever see that low level. On the other hand, if it is being used with high-level signals, it will only ever see those signals. In other words, it's a choice that occurs when the installation is done. Using Telecom relays gives me those options. \$\endgroup\$ Commented Apr 5, 2019 at 21:35

3 Answers 3


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)

  • \$\begingroup\$ Yes, for electromechanical relays, the interruption, not the isolation, is the bottleneck and they are rated as such. \$\endgroup\$
    – DKNguyen
    Commented Apr 5, 2019 at 20:37
  • \$\begingroup\$ Interesting thought. I haven't actually tested the candidate relays yet - I'm waiting to order them. But I have both AC & DC high-voltage breakdown testers (Hi-Pot) here. But more interestingly, the relay isn't OPENING when the spike occurs, it is CLOSING. I suspect there is quite a difference between the two cases. \$\endgroup\$ Commented Apr 5, 2019 at 20:38
  • \$\begingroup\$ Yes, there is a difference as interruption very specifically means opening the relay, not closing \$\endgroup\$
    – DKNguyen
    Commented Apr 5, 2019 at 21:10


simulate this circuit – Schematic created using CircuitLab

In theory with 1k to 3kV/mm for air breakdown smooth contacts, and there is some tolerance for gaps and environmental contamination that may reduce this to 20% of these values.


Use a better, sealed relay rated for 1kV for reliable opeartion.

mercury wetted reed relays are not expensive. 2A 1kV $5typ


Or use a Reed Relay.

  • \$\begingroup\$ Thanks for the link - I've bookmarked it. But my spec requires SPDT contacts. \$\endgroup\$ Commented Apr 5, 2019 at 21:50
  • 1
    \$\begingroup\$ Question to all: Why the down-vote on this answer? There is useful information here. \$\endgroup\$ Commented Apr 5, 2019 at 21:51
  • \$\begingroup\$ Probably someone antagonistic somewhere. \$\endgroup\$ Commented Apr 5, 2019 at 22:12
  • 1
    \$\begingroup\$ +1 .I only downvote answers that are Wrong . \$\endgroup\$
    – Autistic
    Commented Apr 6, 2019 at 0:23
  • \$\begingroup\$ opinions of silent critics may be subjective, biased and often prone to errors \$\endgroup\$ Commented Apr 6, 2019 at 2:59

Magneto service is fairly easy duty for relays. Think about it; with a magneto ignition you leave it open (disconnected) while the engine runs, and close it (short it to ground) to ground out the coil and stop the engine. Think about what that does to relay states.

  • In closing, the contacts may arc as they come together. However the arc is instantly extinguished by the contacts closing in full.
  • Opening will never face a load. Because the engine isn't running if the contact is closed. Ergo very little magneto speed, ergo weak spark. (Normally opening is the worst case for a relay, because an arc will strike as the contacts start to open, and could then sustain once they're fully open. Not an issue here.)
  • Holding closed, the only risk is arcing between contacts and coil or chassis, and that's pretty easy to insulate.
  • Holding open, the contacts need to simply be far enough apart to prevent a cold leap.

Better, magneto voltage acts like AC, because it self-interrupts periodically which means arcs will self-extinguish like AC (better, actually), but again arcing-on-open isn't a big problem here.

Now about double-pole, you only want to do that so you can put the contacts in series for double the voltage resistance. You're giving us an XY problem there; your real problem is you want to interrupt 400V intermittent. So take that on de novo.

I just asked mouser.com for electromechanical -> relays -> contact voltage 400VAC and up. There's a sea of them. It's not a problem.

I know you say you want telecom relays, but that's just you being comfortable with a particular type. That's nice but it is not appropriate here. Honestly, itll probably work - like I say, electrically this is easy service for a relay.

But mechanically, the vibration is very worrisome, and telecom relays are totally not made for high-vibration situations. . I would be looking for vibration resistant relays. Particularly you can't have relays where the vibration closes the relay, because that'll make the engine miss.


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