Common Failure Mechanisms for Submersible Pumps & RCDs

[matheinste]

Hello all.

Does anyone know of a common failure mechanism for submersible pumps whereby they trip household Residual Current Devices. I have had the same problem with three such pumps. I assume the problem is not the fact that they are inductive loads (I assume that they are inductive loads) because they all worked for some time before failing.

They work(ed) on a 240V 50Hz supply and are rated at 300-400W and are fitted with a domestic, pre-wired three pin plug. I do not know the setting of the RCD but the whole supply for the house runs through it.

Thanks for any replies.

Matheinste

 

[dlgoff]

...because they all worked for some time before failing.

Do you mean that the pump failed or that it tripped the RCD?

A residual current device (RCD), similar to a residual current circuit breaker (RCCB), is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor.

http://en.wikipedia.org/wiki/Residual-current_device" [Broken]
The induction of the motor starting is probably causing the inbalance between the line and neutral.

 

[matheinste]

Do you mean that the pump failed or that it tripped the RCD?
http://en.wikipedia.org/wiki/Residual-current_device" [Broken]
The induction of the motor starting is probably causing the inbalance between the line and neutral.

Thanks for your reply.

By failing I mean that the pump caused the RCD to trip and does so immediately it is switched on again. I understand that an inductive load puts current and voltage out of phase and can cause problems with trips but I assumed that as the pumps were fitted with plugs for household use (as cellar sump pumps)that they would be OK in that respect.

Matheinste

 

[dlgoff]

My suggestion would be to get an electrician to take a look at the wiring/circuit that the pump is on to see if all the connections are good. As you say, "the pumps were fitted with plugs for household use (as cellar sump pumps)that they would be OK".

 

[vk6kro]

Do they only fail when they are submerged?

Maybe they are getting water in them?

 

[matheinste]

Do they only fail when they are submerged?

Maybe they are getting water in them?

That is of course a possibility but with such a failure rate any such fault would surely be a design failure and have been flagged up before now. They were being used well above their maximum depth specification and were not running dry.

Although the pumps were wired for household supply, my RCD may not be normal. Perhaps the motor characteristics change with age. I will get more info on the RCD, such as its trip current magnitude and duration. I suppose it may be faulty.

Thanks for your replies.

Matheinste.

 

[vk6kro]

Hi,

If you can, get hold of a high voltage insulation tester and measure the resistance to the casing from the active lead of your motor.
If it is anything but an open circuit, the motor might have water in it.
Even a multimeter on the high ohms scale might give you a reading.

The other thing that can trip an RCD is if the motor has large filter capacitors across the input.
These carry AC current to Earth and can trip an RCD if they are big enough.

 

[matheinste]

Hi,

If you can, get hold of a high voltage insulation tester and measure the resistance to the casing from the active lead of your motor.
If it is anything but an open circuit, the motor might have water in it.
Even a multimeter on the high ohms scale might give you a reading.

The other thing that can trip an RCD is if the motor has large filter capacitors across the input.
These carry AC current to Earth and can trip an RCD if they are big enough.

Yes I have access to a Megger and will do a check to the exposed screw heads in the otherwisw plastic casing. I am an electronics technician by trade and so understand low voltage circuitry but am wary of anything domestic. These things, I know, can have hidden technicalities not always known to an unqualified electrician and this is why, for safety reasons, I asked for help.

I think for now this has gone as far as it can. Thanks for your replies and if I find an answer I will post it.

Matheinste

 

[vk6kro]

The pumps have 3 pin plugs, so just measure from active to Earth on the plug.
Glad you have a megger. Set it on minimum voltage (250 V ?) so you don't generate any new faults.