Electricians of STW

Our neighbourhood had a power surge a few months ago and it blew our sewage pump. Its been replaced and had the wires checked. Nothing else in the house was affected.

The RCBO (30A) on the consumer unit randomly trips and we cant work out why. Sometimes its fine for weeks and other times it trips twice in one day. There is a 13A fuse between the pump and consumer unit that has been fine so surprised the 30A RCBO is tripping.

The CU has an RCD circuit with MCBs for upstairs. Another RCD circuit with MCBs for downstairs. Then (not on the RCD circuits) an RCBO for the pump and another MCB for a workshop.

The pump says it will pull up to 15amps under load but as said above has never blown the 13amp fuse

Before we call out an electrician, is there anything silly im missing?

RCBO will trip for overcurrent and earth leakage. Unlikely to be overload on a 30A breaker, so earth leakage is most likely culprit, followed by faulty RCBO.

RCBO are essentially a combined RCD and MCB, so it could be an earth leakage fault, or it could just be faulty. It could also be that the new pump has a higher start-up current demand.

Unless you have an RCD tester (and probably a Megger/Insulation tester), you’re probably going to need an electrician to narrow down the problem.

As for fuses. The 13A rating is the continuous rating. For the fuse to blow, it requires being overloaded for a period of time. The higher the overload, the quicker it will blow, but unless it’s a big overload, it can take many seconds for a fuse to blow (I’m sure a bit googling will turn up the BS spec for domestic fuses).

A MCB/RCBO is likely to trip faster, depending on what Type it is. Type A will pretty much trip instantly on overload, whereas Type B and C have increased overload thresholds, to allow for things like motors that have a high start-up surge.

As above also has damp got into any of the connections.

Water getting in somewhere. It’ll be tripping the RCD portion of the RCBO, not the over-current portion.

Check all connections, it could even be water inside the pump, getting past the seals.

The RCBO has already been replaced to rule that out.

There are 2 junction boxes I can check for damp. 1 above ground with the 13a fuse and 1 inside the septic tank. Both are new and easy enough to check.

If water is getting in somewhere would it not trip all tehe time amd get worse? Its completely random and sometimes can be fine for days/weeks.

The pump is pretty big. About 30kg and fully submerged so id have thought if any damp has got in it would keep getting worse and just stop altogether.

How often does the pump operate normally?

Is the chamber that the pump stands in clean?

Does the RCBO reset easily, first time?

Large motors frequently require a type c mdb. The initial resistance of the windings is so low there can be a higher current spike stauration occurs

Is there a float switch or similar that is separate to the pump?

The pump sits in a small septic tank so its not clean, but the the pump is designed to sit in there.

It operates every 2 days normally. Sometimes once a day.

It has a type B RCBO that resets first time.

It has a float switch that came with it on about a 40cm cable. My guess is at times the water level rises slowly and the float switch might be turning on slowly. A bit like pressing a light switch slowly, and the RCBO sensing a loose connection?

The pump sits in a small septic tank so its not clean, but the the pump is designed to sit in there.

I’m thinking about excessive/wrong waste rather than that needed to maintain bacteria and a natural process, however, the fact that it resets easily tends to rule out a mechanical blockage and resulting high inrush current

Electrician then, but an intermittent fault might take some finding

What type of RCB0?

Motors/pumps should be type C or D, if it’s a type B then it could trip out when the pump starts up due to high starting currents.

Here’s a simple guide to mcb/rcbo breaker types.

https://uk.rs-online.com/web/content/discovery/ideas-and-advice/mcb-guide

type B RCBO

There’s your problem. See my post above.

How about more veg and less carbs/ protein? Or a type C breaker. Hopefully you get it fixed.

Had a crackly plug at our place on the kettle socket that I thankfully found before it burnt the house down. That along with a spilt glass of wine on the carpet and the dog doing number 1 and 2 on the landing carpet has given us an eventful Christmas.

There’s your problem. See my post above.

If the circuit was designed for a type-B RCBO then that’s what it should have.

It might take a type-C as is, but a type-D is a probable redesign to account for the earth fault loop impedance. In either case the data must come from the manufacturer of the RCBO for accurate calcs

If it’s worked for years in type-B configuration then there will be a fault somewhere so get an electrician in

How certain are you the fault is in the the pump circuit?  Intermittent earth leakage faults are fairly common (and a PITA to trace).  If you’ve not already I would turn off the circuit to the pump, and manually turn it back on for brief periods each day to allow it to do its job.  If nothing else trips during say a week of wet weather then you may have  confirmed the problem – but it may be coincidence this has appeared after a pump swap.  Or I believe they are summative – so if you are losing 10 mA through the pump circuit, 10 mA through a washing machine and 8mA through an outside light you will only need 2mA through something else to trip it!

How certain are you the fault is in the the pump circuit? 

Only the pump is connected to that RCBO. I’d guess if the problem was on a different cir uit it would trip a different breaker.

Theres also no way to know when the pump is running while iside the house or over night, out the house etc so can’t really monitor exactly when it trips.

I did a bit of inspecting yesterday and there was a little moisture inside the junction box in the tank. The junction box is 110 x 110 x 50mm and had a few droplets of water in. I’ll be changing that box for something much smaller with hopefully more resistence to moisture. I really hope this is the issue.