Fishpond pump trips RCD (not really)

[crappy]

Hi Folks,

I have a fishpond pump which also has a UV lamp. These two have been running for months without any problem and still don't have a problem. The odd thing is though, I installed a timer and whenever the timer cuts out, the RCD drops. Almost without fail this will happen.
I recently turned off the main power as I was working on something and when I turned it back on, the RCD was down again.

The timer is a simple SPDT relay which interrupts live. Neutral is always connected.

Once reset, everything is happy again.


The RCD is built into the double outside plug.

No amount of switching the plug on and off can reproduce this - only when power is cut.


Why would this happen? (Some sort of back-EMF from pump / CF tube?)
How can I stop it as the pump will not turn on again.


Cheers,
Crispin

 

[JohnW2]

....The odd thing is though, I installed a timer and whenever the timer cuts out, the RCD drops. Almost without fail this will happen. .... I recently turned off the main power as I was working on something and when I turned it back on, the RCD was down again.
The timer is a simple SPDT relay which interrupts live. Neutral is always connected. Once reset, everything is happy again. The RCD is built into the double outside plug. No amount of switching the plug on and off can reproduce this - only when power is cut.

I think what you're seeing is the normal behaviour of an 'active' RCD - the type which is usually built into sockets or plug-in RCDs. Unlike the 'ordinary' sort of RCDs ('passive') you find in a consumer unit etc., active RCDs are designed to trip when power is lost. This is a safety feature of particular relevence to power tools, garden tools etc.,since it avoids the tools coming back into operation when power is restored (until the RCD is reset).

To avoid this, your timer relay would have to interupt the supply after the RCD (or find a socket/plug with a passive RCD, if they exist).

Kind Regards, John

 

[crappy]

Thanks John,

I did not know there are two types. Seems you are correct (if not the wrong way round though? )

From Wikipedia

Active/passive latching/non-latching
RCDs may be obtained that have different behaviours if the circuit they are protecting is de-energised.
One type will trip on power failure and not re-make the circuit when the circuit is re-energised. This type is known as non-latching.[citation needed] [8]
Another type will maintain the circuit after power is restored. This type is known as latching or active.
The first type is used when the power-drawing equipment is regarded as a safety hazard if it is unexpectedly re-energised after a power failure e.g. lawn-mowers and hedge trimmers.[citation needed]
The second type may be used on equipment where unexpected re-energisation after a power failure is not a hazard. An example may be the use of an RCD on a circuit providing power to a food freezer, where having to reset an RCD after a power failure may be inconvenient.

Active will retain the circuit after power trip. (or is wikipedia wrong?)


So basically, I'm scr....ed.

Bugger

 

[EFLImpudence]

(or is wikipedia wrong?)

John is correct.

Passive = doesn't do anything. That is in the event of power loss.

 

[crappy]

makes more sense that way round.

Imagine, WP is wrong on something

 

[JohnW2]

Thanks John, I did not know there are two types. Seems you are correct (if not the wrong way round though? )
From Wikipedia .... Active will retain the circuit after power trip. (or is wikipedia wrong?)

I think the wikipedia is wrong.

So basically, I'm scr....ed. b*****r

Well, I'm sure there are workarounds. Firstly, as I said, if you could do your time-switching after the RCD, that would solve the problem. So would finding a 'passive RCD' device to replace your one (if the exist).

Is the socket in question fed from a circuit which has 30mA RCD protection (in your consumer unit)? If so, you don't need the second RCD, so you could replace the RCD socket or plug (you said plug, but I wonder if you mean socket?)with an ordinary one.

Kind Regards, John.

 

[crappy]

sorry, I did mean socket. Built into the two gang jobbies.

The switching is post RCD - it's a plug in timer.

There is no RCD in the CU (still confused as to why it is not law - it is in other parts).
I've been wanting to put one in for a while - maybe I should just do that now. Having one on the feed has probably saved my bacon a few times over the years (kids should not play with electricity )

So actually, now that brain is kinda working, my switching being after the RCD, that discounts the active / passive thing?

 

[JohnW2]

sorry, I did mean socket. Built into the two gang jobbies.
The switching is post RCD - it's a plug in timer. So actually, now that brain is kinda working, my switching being after the RCD, that discounts the active / passive thing?

Yes, that rather puts paid to my 'clever' answer - although the fact that it's an active RCD does at least explain why the RCD trips when you turn off the main power!

More thinking required!

Kind Regards, John.

 

[JohnW2]

More thinking require!

I guess you're going to have to go through a process of elimination.... Have you tried plugging something else into the timeswitch and then seeing if the RCD trips when the timeswitch 'switches off'?

Kind Regards, John

 

[davelx]

As an aside - you can get both passive and active RCD socket outlets, so if you want the pump etc. to continue to run after a power cut, you can change the outlet to a passive version.

This doesn't fix the other issue, of course.

 

[crappy]

I've not tried a different load on it - will do that today. I've confused by the fact that it works fine all the time.


It is the [enter correct name] version as again today I turned off the mains and it tripped.

Will stick another load on and update.

 

[JohnW2]

I've not tried a different load on it - will do that today. I've confused by the fact that it works fine all the time.

It is confusing, but funny things have been known to occur at switch-off. If the load is such that (either in normal operation or due to a fault), a small current flows in the neutral after live has been disconnected, that can trip an RCD. Hence my suggested experiment, to help narrow things down.

It is the [enter correct name] version as again today I turned off the mains and it tripped.

What you have is an active RCD in your socket

Kind Regards, John

 

[crappy]

Well, since we last spoke it has behaved itself.

There is a waterproof junction box under the deck which _could_ have got some moisture in. I would have expected it to trip "randomly"

Ahh well, all is well for now. (I might try fish out the 'box at some point)

 

[Steve]

Dont have the pond pump/filter on a timer. Turning it off causes bacteria to multiply in both, to dangerous levels. When you turn it back on these dangerous toxins get into the pond and are no good for the fish. It'll work more effectively if it runs 24/7.

 

[crappy]

Dont have the pond pump/filter on a timer. Turning it off causes bacteria to multiply in both, to dangerous levels. When you turn it back on these dangerous toxins get into the pond and are no good for the fish. It'll work more effectively if it runs 24/7.

thanks for the tip -
interestingly, and quite OT , it has gone from its former crystal clear self to fugly green. Think it needs to go back to normal...


/OT