Mains power restored alarm, non contact

If you have installed a transfer switch and have access to a backup power source such as a generator or a battery bank and mains inverter, then you can safely switch power for the whole property between a grid supply, and your backup source. The transfer switch does this safely by ensuring the power transfer is "break before make" - forcing the disconnection of one source before the other can connect. This ensures there is no danger of your local power source attempting to back feed the grid supply - something that at best would overload your local generation capability, and at worst could kill or injure someone working on the grid supply.

One shortcoming of using a transfer switch is that once the grid connection is disconnected, how can you tell when the power cut is over? This article explores a small project to make identifying when the grid mains power is restored, easy!

Detecting mains power

When operating from your own power source, typically all of your installation is disconnected from the grid supply, and so checking if it working is more complicated just just plugging something in, or turning on a light. If you have an electronic electricity meter which has an integral LCD display, you may be able to see when grid power is "on" because the display will be functioning. However this is something you need to go actually "look at" to see. It would be nicer to have some kind of audible alarm that notify you when the power comes back.

Direct connection

One way to detect power is to make a direct connection to the incoming grid supply terminals on your transfer switch, since these will go live whenever the grid supply is working even if you are not currently switched to that supply. The difficulty here is that these terminals are typically before all of your local circuit protection such as the fuses or circuit breakers in your consumer unit. The only protection available here is that provided by your main incomer fuse (typically at 60A or 100A). Connecting to that typically requires meter tail sized conductors (usually 16mm² or 25mm² cables). So to safely make connection here, you would need a set of tails to feed a dedicated small CU containing a MCB or RCBO. Lots of extra complexity and expense. Alternatively you could for example install a simple neon indicator on the understanding that it is very low risk, but again you then have something that needs looking at periodically.

Non contact connection

If one can detect mains presence without making direct contact, this all gets simpler. Non contact power detection can be done by inductive or capacitive coupling to a live wire. Inductive coupling (often using a current transformer with a coil placed around a wire) can detect when current flows in that wire. However we can't use that there since there will be no load on the supply wiring at this point, and so no current flow to detect.

Capacitive coupling however will work, since this will allow the presence of a voltage to be detected even when there is no current flow. This is the same principle used by commercial "volt stick" style detectors. (in fact if you have one of these that can run for hours and not automatically turn off, then spring into life and beep at you when the power comes back on, you can stop reading the rest of this page now!)

Theory of operation

Whenever you place two isolated conductors in close proximity, any voltage applied to one will tend to cause an accumulation of electric charge on it, and that in turn will result in an opposing electrical charge accumulating on the other conductor, forming a capacitor. This accumulation of charge on one side of the capacitor will briefly allow a current to flow into it as it "charges". However once charged, that current flow stops. You can however allow the charge to flow out again, so you will see a reverse flow of current as the capacitor discharges. While a steady direct current can't flow past a capacitor, alternating current can - since the continuous reversals of current flow direction can repeatedly charge and discharge the capacitor.