New CU keeps tripping

[dx90]

I guess he will be replacing the 100mA RCD with a Time Delayed (S Type)?

Why would replacing the 100mA with a timed delayed type help?

Yes we got the bill and sent off the check the same day.

 

[Spark123]

I'm in the frame of mind of it being something on the 100mA side, if it was on the 30mA side I'd have expected it to trip the 30mA RCD or both of them. You say lighting, boiler and cooker on the 100mA side? Which of these circuits are in use when the trip occurs? Do you have outside lights? Any sign of water ingress? Replacing with a Selective (S) RCD will help them to discriminate by the (S) RCD giving the normal type RCD time to clear the fault before the (S) RCD starts to trip. You say the socket MCB also trips - I take it there are a few power hungry items plugged in which all fire up at the same time when the RCD is reset?

 

[dx90]

What sockets are on the down radial are only the living room ones. The dinning room is on the kitchen ring main and the kitchen is on a separate MCB.

I have 4 items that come on and go to standby, recorder cable box etc. I do have a 28 inch flat screen CRT TV that does come on when trip is reset. That thing must take a heavy load when first switched on. Hopefully this explains about this one tripping when turned back on.

The cooker on one occasion was on 20 minutes prier to tripping but was off when it tripped. Other time it’s been off for hours.

The combi wall boiler only comes on when we call for hot water (washing machine, kitchen sink or bathroom). One of these might have been used when some of the trips occurred I didn't check, but I'm sure it wasn't on when it last tripped.

The lights up and down are on the same ring main, last time it tripped they where on but other times it was in the day time and they where off.

We do not have any out side lights.

Thinking back I would say that any one of the three could have been on at any trip but not always the same one and not all at the same time because of the time of day. There doesn’t seam to be a pattern to the tripping.

 

[dx90]

Spark123 when you said about replacing with a Selective (S) RCD will help them to discriminate by the (S) RCD giving the normal type RCD time to clear the fault before the (S) RCD starts to trip.

Could you explain to me if the 100mA does get changed over for the S version what should happen different with the tripping to what is happening know?

Is the S version a direct replacement for the standard 100mA RCD or does the S version go inline between the meter and the present 100Ma RCD?

Thanks

 

[JohnD]

The time-delayed version should fit in as a direct replacement.

because it is slower to trip, any circuit which is on the 30mA RCD side of a split load,or is on an RCBO, and has an earth fault, will trip faster, so you will only lose power to the affected circuit(s) and not the whole house.

The only time when this does not hold good is when you have (say) ten circuits, each with a small background leakage of (say) 20mA, which adds up to enough to trip the S-type.

Note that some appliance with earth leakage (especially watery ones) may have a neutral-earth current even when they are switched off/not in use. So it is necessary to unplug them, or switch off with a DP isolator, to eliminate them as potential causes. A small water leak in a washing machine, kettle, boiler, central heating pump or shower which gets onto an electrical part can seem to be random, as can any appliance controlled by a timer or thermostat. Heating elements, especially in cookers and immersion heaters, often develops slight earth leakage as they age.

 

[dx90]

Can I check for earth leakage myself with my multi meter?
What setting would I use?

 

[securespark]

Yes

Can I check for earth leakage myself with my multi meter?
What setting would I use?

The best way is with a clamp meter that can measure mA.

Clamp it round the main earthing conductor & see what the total drain is. trouble is the bits of kit are expensive - maybe you could hire one?

While you're at it, check the RCD's to find out what level of fault current they trip at.

 

[dx90]

The multi meter that I have is called a Precision Gold WG 022 from Maplins that has mA on it. Can I get some clamp leads for it and use this for testing?

If I have to get a Digital Clamp meter to test, can anyone recomend one that works ok and is one of the cheaper ones if possible?

 

[bernardgreen]

Clamp it round the main earthing conductor & see what the total drain is. trouble is the bits of kit are expensive - maybe you could hire one?

Meausring current in the main earthing conductor may not give a true value of earth leakage as paralllel paths to earth may be carrying some or most of the earth leakage current.

Putting the clamp meter round live and neutral together should show the difference in currents in those those wires. That difference, which is the true leakage for that circuit, is the parameter the RCD mechanism responds to.

 

[dx90]

I've been told is it’s an accumulative effect from different sources from some of the circuits, not enough to trip the 30mA RCD but enough to trip the 100mA RCD. I think the reading was about 40 when measured on earth at CU and he gave the impression that it must be rising at some point to trip the 100mA RCD.
I did ask about an S version RCD and he said that in this case it would only delay the trip and nothing else.

Thanks bernardgreen for the info.

Is there a way I can check an appliance for earth leakage before and after a repair?

 

[bernardgreen]

Is there a way I can check an appliance for earth leakage after a repair?

I used to have a test rig used for for this and, in a slightly different form, for measuring earth leakage on comms equipment to pre-empt failures.

Current transformer with Live Neutral through it. Output to a resistor and the voltage across this gave an indication of the leakage deduced from the difference between Live ad Neutral currents. Not very accurate but it gave a trend that indicated developing faults.

To get a more accurate measurement a third wire was put through the current transformer . An AC current was sent through this wire and adjusted until the current transformer gave a zero output. The current in the offset wire was then equal and opposite to the earth leakage difference.

 

[dx90]

It sounds very complicated to build.
Is there a device that can be bought that plugs inline between the appliance and the socket?

 

[Spark123]

I've been told is it’s an accumulative effect from different sources on different MCB’s, not enough to trip each 30mA MCB but enough to trip the 100mA RCD. I think the reading was about 40 when measured on earth at CU and he gave the impression that it must be rising at some point to trip the 100mA.
I did ask about an S version RCD and he said that in this case it would only delay the trip and nothing else.

Thanks bernardgreen for the info.

Is there a way I can check an appliance for earth leakage before and after a repair?

I thought you said you had a 100mA and a 30mA RCD in a split load CU? An RCD and an MCB are two different devices (it is highly unlikely you have 30mA MCBs.) There is a device called an RCBO which incorporates an RCD and an MCB, do you have these? Can you post a photo of the CU?

 

[dx90]

Hi Spark123
Apologies for mistake in my post. I have corrected it know. unfortunately I don't own a camera or know anyone that could loan me one.

The RCD reads 80A-100mA WRM80/2 and the other RCD 80A 30mA WRS80/2 on the 100mA side is the cooker NSB40, lights NSB06 and combi boiler NSB16 and on the other 30mA side is sockets up NSB16 and down NSB16, kitchen sockets NSB32, shower NSB40 all are Wylex.

 

[Spark123]

If it isn't enough to trip the 30mA and all the sockets are on the 30mA side it is unlikely to me to be a portable appliance causing the problem.