The rainwater harvesting pump stopped working yesterday evening.

After a very dry spring and with a newly-planted hedge and a newly-seeded area of grass needing a significant amount of watering, the underground rainwater tank had emptied from a fully-filled 5,000 litres on 30th March to 1,300 litres on 26h April, with only 900 litres being replenished from rainfall around the middle of April.

After about 20 minutes of watering the grass seed using a hosepipe at around 8pm yesterday – while also running the ‘soaker’ hose along the hedge line – the water stopped flowing. First thought was that the pump had taken objection to running constantly for so long (though it had run for rather longer previously without any issues) but upon checking the pump controller it’s LEDs were all off; normally at least the green ‘Power’ LED is on.

Next thing to check was the circuit breaker (a dedicated RCBO, only supplying the Rainwater Harvesting controls) – which had tripped. Upon resetting that it immediately tripped again. By that time it was getting late so problem solving was left until the morning.

I’d made the decision to leave the supplied German plugs on all the RWH control units, connecting those to ‘Shucko’ socket inserts installed by the electricians, fed from the dedicated 16A RCBO. That made it easy to disconnect those plugs to try to isolate the issue (compared with if the plugs had been removed and the connections hard-wired). With all the plugs out there was no fault evident and the circuit breaker stayed ‘on’.

A quick check of the Pump cabling showed Live and Neutral had a suitably high resistance to Earth, though only about 5.5 Ohms between the two of them – which seemed a bit low, but not ridiculous for an electric motor. After reconnecting the Pump Controller and the Pump itself, the electrics were behaving fine but the pump wasn’t pumping. The red ‘Failure’ LED was lit on the Pump Controller, which is believed to be an indication the Pump had failed to supply pressure when powered up by the Controller. (I’ll come back to this later.)

There’s a second Schuko socket for the mains water top-up. That has a plug wired directly to the solenoid valve for the mains water supply and this fits into a special ‘adaptor’ which is wired to a float switch in the underground tank. The idea is that the valve stays unenergised (i.e. blocking the water flow) until the float switch closes (at low water level) – at which point the adaptor is made live and the valve opens. Plugging in the adaptor made the circuit breaker trip again. Checking the wiring inside the adaptor showed evidence of a short from Live to Earth, whenever the float switch was ‘closed’.

That explained the failure scenario:

• With just over the minimum level of water in the tank everything was working normally
• As water was used up, the tank level reached the minimum and the float switch switched – causing a short-circuit in the adaptor and tripping the breaker, thus removing power from the pump and stopping the water flow

The underlying issue turned out to be that the Adaptor has two sprung ‘clips’ which provide the Earth contact on the top and bottom of the inserted Plug. These clips move ‘outwards’ when the Plug is inserted and the lower one was very close to the wiring entering the Adaptor at the bottom. Evidently this clip placing pressure on a wire was enough (over time) to compromise the insulation and cause a short circuit. (It was the Blue wire which had failed. Normally that would be Neutral but here it’s acting as Switched Live.) The resolution was to leave none of the outer cable sheathing projecting beyond the cord grip, allowing the wires to be routed further away from the Earth clips, after the damaged wiring was cut off and re-terminated.

With the adaptor re-wired and re-fitted, the breaker stayed closed and the mains water top-up valve opened to add more water to the tank. I let it fill from 1,300 litres to 2,300 litres, which was enough to fully submerge the float switch (though that was still ‘closed’ – which is a separate problem).

I’m not entirely sure if the issue with the pump was caused by me removing it from the tank for inspection (letting air in and causing issues with it re-priming) or if it had separately had problems due to the low level of water in the tank. Even with an extra 1,000 litres of mains top-up it wasn’t managing to deliver any pressure when the Pump Controller powered it up. I could hear it trying to start pumping when the controller switched it on – I think it was spinning but ‘dry’ – but the overheat protection then kicked in (normally it gets cooled by water passing through its casing, past the motor windings). Tipping it over onto its side at the bottom of the tank let the air bubble out of the bottom, and it then started OK while still horizontal – and kept running when turned vertical again. I’m glad I left a ‘loop’ of pressure hose in the supply line to enable the pump to lie flat.

I’m still not sure what’s wrong with the float switch – which appears to be permanently ‘on’. Perhaps the switch contacts ‘welded’ closed due to the short circuit, before the breaker tripped (three times)? Wikipedia says the actual switching element is typically a regular microswitch, operated by a lever and a rolling steel ball (which you can hear). For now it’s easy enough to unplug the feed to the solenoid valve and keep an eye on the water level manually; maybe it will fix itself. There doesn’t appear to be any way to access the internals of the float switch without destroying its casing.

There’s finally been a bit of rain this afternoon. Only enough to add 400 litres to the tank – but at least the hedge and the grass seed don’t need watering today.