men in sheds

[Stueeee]

If you search for 'gas interlock controller', this will do what you want. They are used in commercial kitchens with a solenoid valve on the gas line such that if the fan is not running, the gas does not flow. They work on either current sensing or pressure sensing on the fan. You would not use a gas solenoid valve, just a contactor on the supply to the machines: no fan, no juice to the machine.

One Shed I help out with has a simple system.
By each machine has a extractor attached, switched on at the wall nothing goes life.
But when the machine is stopped the extraction kicks in.

There was a few "older" members who thought H&S was not for them.

They got educated.

Interesting ideas, thanks for those. I found myself getting "involved" in this issue by one of the shed members (not Sue) who had bought a 'Record Power' automatic switch unit for their shed. This works by sensing that a machine had been turned on thereby switching in the extractor which is trunked to each machine around the workshop. The chap who bought it not unreasonably expected it to work their existing Record power twin motor extractor, but it turned out to have a maximum power budget of 2000W as opposed to the around 4 KW-ish load of the extractor, so no use for their purpose.

Being as they appeared to be stuck with this device I wired a contactor coil so that the "extractor" outlet operated it but with the extractor power coming from a second 230V source to extractor via the contactor's contacts. After some tuning (poor sensitivity of the Record box required a loading resistor in parallel to the contactor coil) this worked OK until the Record box went phut just after the warranty expired.

As it's clear that the Record Power box is a something of a chocolate teapot, it didn't seem to be worth replacing it. I came up with a fairly simple solution that adds a small mains relay to each saw etc. machine; the make of the first contact pair operates the main contactor which then starts the extraction, and the contactor will hold operated until it's released when the last machine is turned off. The relays fitted to each machine have a pair of make contacts which are paralleled with the contacts in all the other machine specific relays. The main contactor will be fitted with a timer to give a short overun of the extractor when the last machine has been turned off.