O
Octopus
what would happen if you plugged a kettle into said circuit and specifically the same plug the hood is using? The fact that 2 different hoods tripped the RCD does "suggest" a circuit fault.
Discuss Cooker hood tripping RCBO - head scratcher in the UK Electrical Forum area at ElectriciansForums.net
Share this page
what would happen if you plugged a kettle into said circuit and specifically the same plug the hood is using? The fact that 2 different hoods tripped the RCD does "suggest" a circuit fault.
Appreciate everyones input.
@Murdoch
Said socket tests satisfactory - renewed it anyway.
As well the normal tests plugged in a 2.5 KW heater which had no issues.
Basically when on a short radial circuit hood trips -whether on short leg of ring or ccu socket (cooker circuit approx same length).
@Murdoch
Said socket tests satisfactory - renewed it anyway.
As well the normal tests plugged in a 2.5 KW heater which had no issues.
Basically when on a short radial circuit hood trips -whether on short leg of ring or ccu socket (cooker circuit approx same length).
O
Octopus
- Reaction score
- 4,325
Suggest you swap the 13amp socket for an MK filter socket. And also consider swapping TO RCBOs which are not susceptible to transients - Hager range for example.
Problem is all to do with the way the cooker fan motor speed is switched and controlled plus marked imbalance in length between short and long sections of ring main from cooker hood socket to RCBO.
I spend an hour writing out an explanation but have lost it and auto-save has not rescued.
Problem is all to do with the way the cooker fan motor speed is switched and controlled plus marked imbalance in length between short and long sections of ring main from cooker hood socket to RCBO.
I spend an hour writing out an explanation but have lost it and auto-save has not rescued.
Last edited:
Hi,
No update as Bosch have only just sent an engineer and have been recovering from a carpel tunnel op - apologies.
Bosch will only say a class A RCBO Might solve the problem - difficulty being there was only room for a 4-way board in the small meter cupboard & I can only find double pole class A RCBO which won't fit. Checked the manual & no mention of this issue.
Marconi, thanks & will try a filtered socket though the board is all RCBO - be interested in that explanation and found vague references to it when I originally searched.(good to exercise the old grey matter).
As I am getting back to work another workaround would to be feed the hood from a spur from another leg of the ring main - way cheaper than swapping out RCBO's etc- can't think why this wouldn't be ok as it's not really a fault as such - thoughts?
Thanks.
No update as Bosch have only just sent an engineer and have been recovering from a carpel tunnel op - apologies.
Bosch will only say a class A RCBO Might solve the problem - difficulty being there was only room for a 4-way board in the small meter cupboard & I can only find double pole class A RCBO which won't fit. Checked the manual & no mention of this issue.
Marconi, thanks & will try a filtered socket though the board is all RCBO - be interested in that explanation and found vague references to it when I originally searched.(good to exercise the old grey matter).
As I am getting back to work another workaround would to be feed the hood from a spur from another leg of the ring main - way cheaper than swapping out RCBO's etc- can't think why this wouldn't be ok as it's not really a fault as such - thoughts?
Thanks.
Leesparkykent
Esteemed
- Reaction score
- 9,113
You can get type A RCBO's. Most RCBO's with the cream functional/reference earth lead are type A.
Last edited:
- Reaction score
- 4,325
Been back in the trade a few years now & a regular lurker here (236 I & II & C, 2391, time served etc, 17th) - I have always managed to track down answers using the search however need some help on this one.
New kitchen install in a 2 bed bungalow involving a board swap to RCBO's as only three circuits (lights, ring & cooker).
2nd fix day went fine, all tests carried out with satisfactory results - cooker hood flicked off & on to check function before leaving - job done.
Call via kitchen company I'm subbing for that RCBO is tripping out when fan speed increased on cooker fault. Go back & double check IR tests (all over 200 MOhms), test RCBO again (X1 19.7 X5 7.8 ramped at 27 mS).
Earth leakage clamped & when fan speed increased see a jump to 70 mA - aha I think faulty cooker hood.
Replacement arrives but fitters call to say same thing happening.
Go back again & found that if I connect hood to short leg of ring back to board it trips - connect it to other longer side of ring it doesn't! So remove all other circuits from board & same thing happens. Then swap over to similar length cooker circuit (2M) & it trips! Connect to short leg via extension lead it doesn't trip. This all after double checking IR etc. Swapped RCBO's , double checked connections in consumer unit.
I have installed numerous kitchens recently & never seen this fault. I realise that the common factor here is the short length of the circuit but can't work out why a common Bosch cooker hood would generate 70mA leakage because of it!
By Marconi This is what I think is happening based on the information provided. I have highlighted above the key statements which I have considered. I have to make some assumptions, for example I reckon the fan speed is adjusted by switching in/out capacitors which are connected in series with the motor: The sequence of switching is OFF/SLOW/MEDIUM/FAST corresponding to a progressive reduction in series capacitance. I suspect also that the cooker hood socket may be the first socket in the short 2m leg and the supply is TNC-S although the reasoning holds for TN-S. Studying my home's cooker hood, the switch operation is a slider mechanism: I reckon the contacts are break before make but not fast in opening and closing so arcing is possible/likely but short lived.
When the fan is turned to the SLOW position (- as in the quick test of operation mentioned above) the transient current through the path L-N is balanced thus the RCBO does not operate. The motor is spinning and current is flowing through its windings (which are inductive). Energy is stored in the magnetic field of the windings and there is a back EMF close to supply voltage less that dropped across the capacitor and any resistance. This is a steady state of operation. All along the windings of the motor there is capacitance to the earthed metalwork of the armature: at mains voltage the earth leakage is small and much less than would trip the RCBO.
When the fan is switched up in speed to MEDIUM, the situation is once again transient as the motor speeds up to a higher steady state speed. The switch opens before closing the contact to the next smaller value series capacitor. In this short interval, the applied EMF of the mains is momentarily removed which is driving the current through the motor and creating the magnetic field. The magnetic field starts to collapse thereby inducing an EMF in the winding of the motor which acts to maintain the flow of current L-N. This EMF is in series with the supply EMF: a large voltage across the contacts causes an arc to form, which is drawn as the contacts separate and extinguished. The windings EMF is now effectively connected across the capacitance of the winding to earthed metalwork. The sudden step change in potential difference across this distributed capacitance resulting from the opening of the switch - is now the EMF induced by the collapsing magnetic field, which is much higher than the applied mains (and corresponding back EMF of steady state operation). This step change has frequency components much higher than 50Hz so even though the distributed capacitance my not be large, at these frequencies the capacitive reactance is low - large high frequency current (ie 70mA) flow in the series loop formed by winding, distributed capacitance, earthed frame, CPC, CPC-N connection (Local if TNC-S or remote if TN-S), N back through RCBO and back to winding. There is thus an unbalanced current passing through the RCBO - no similar current in L through RCBO- which it detects and causes it operate.
When the contact eventually makes for MEDIUM, the mains supply is reconnected to the motor windings which causes a further transient, again in voltage across the winding and its distributed capacitance and there is once again current flow through the winding capacitance but now through a series path formed by L, winding, distributed capacitance, earthed frame, CPC, CPC-N connection - no similar current through N of the RCBO this time. Making the contacts also establishes again an L-N loop current through the windings which restores a new steady state at higher speed.
So why does the length of cable between the fan and the RCBO matter? The first thing to realise is that Twin and Earth Cable has different electrical characteristics depending on frequency. All quoted conductances and insulation resistances for example are at DC and low frequencies. At higher frequencies something called skin effect matters; the tendency of the current to flow only in the outer perimeter of the conductor which means at higher frequencies the resistance is higher (conductance is lower). It is why lightning conductors are generally broad tapes of copper (or hollow copper tubes) to create the maximum perimeter area for conductance of the lightning pulse. Thus a longer length of T&E will, at high frequencies, have a higher overall resistance end-to-end.
Next, the way the conductors are arranged in T&E, L-CPC-N, means there is more capacitance between the live conductors and CPC than between the live conductors. A capacitor relies on an insulator between to conducting plates - a dialectric material. A dialectric material is not a perfect insulator hence insulation resistance. The insulation resistance of a long length of T&E is lower than for a short length and dialectrics become weaker as frequency increases thus capacitance decreases.
Earlier I explained how I think in moving from SLOW to MEDIUM SPEED a short pulse of current flows in an N-CPC loop via the RCBO when the switch opens and then another short pulse of current via a L-CPC loop when the switch closes.
These currents are of course caused by short pulses of voltage, the former applied within the N-CPC loop and the latter applied within the L-CPC loop.
For the short length of T&E, there is not as much skin resistance and the impedance between its N conductor and CPC is higher so the pulse of voltage is effectively applied across the N-CPC connection via the N of the RCBO. Along the longer length of T&E there is more skin resistance and lower impedance between the N and CPC which means when the fan is connected only to the longer length of T&E, insufficient is present across the N-CPC connection via the RCBO N path for it to operate - the major part of the current in the loop flows between the conductors of the T&E. Similar reasoning can explain a higher current in the L-CPC loop for the short length of T&E.
A further thought on why the longer length attenuates the pulses of voltage is there are almost certainly more sockets in the longer length of ring with loads connected which will attenuate further the pulses propagating back towards the RCBO when the fan switch is operated SLOW-MEDIUM.
These are my musings on this head-scratcher. Without testing and recording some results for analysis that is all they can be.
New kitchen install in a 2 bed bungalow involving a board swap to RCBO's as only three circuits (lights, ring & cooker).
2nd fix day went fine, all tests carried out with satisfactory results - cooker hood flicked off & on to check function before leaving - job done.
Call via kitchen company I'm subbing for that RCBO is tripping out when fan speed increased on cooker fault. Go back & double check IR tests (all over 200 MOhms), test RCBO again (X1 19.7 X5 7.8 ramped at 27 mS).
Earth leakage clamped & when fan speed increased see a jump to 70 mA - aha I think faulty cooker hood.
Replacement arrives but fitters call to say same thing happening.
Go back again & found that if I connect hood to short leg of ring back to board it trips - connect it to other longer side of ring it doesn't! So remove all other circuits from board & same thing happens. Then swap over to similar length cooker circuit (2M) & it trips! Connect to short leg via extension lead it doesn't trip. This all after double checking IR etc. Swapped RCBO's , double checked connections in consumer unit.
I have installed numerous kitchens recently & never seen this fault. I realise that the common factor here is the short length of the circuit but can't work out why a common Bosch cooker hood would generate 70mA leakage because of it!
By Marconi This is what I think is happening based on the information provided. I have highlighted above the key statements which I have considered. I have to make some assumptions, for example I reckon the fan speed is adjusted by switching in/out capacitors which are connected in series with the motor: The sequence of switching is OFF/SLOW/MEDIUM/FAST corresponding to a progressive reduction in series capacitance. I suspect also that the cooker hood socket may be the first socket in the short 2m leg and the supply is TNC-S although the reasoning holds for TN-S. Studying my home's cooker hood, the switch operation is a slider mechanism: I reckon the contacts are break before make but not fast in opening and closing so arcing is possible/likely but short lived.
When the fan is turned to the SLOW position (- as in the quick test of operation mentioned above) the transient current through the path L-N is balanced thus the RCBO does not operate. The motor is spinning and current is flowing through its windings (which are inductive). Energy is stored in the magnetic field of the windings and there is a back EMF close to supply voltage less that dropped across the capacitor and any resistance. This is a steady state of operation. All along the windings of the motor there is capacitance to the earthed metalwork of the armature: at mains voltage the earth leakage is small and much less than would trip the RCBO.
When the fan is switched up in speed to MEDIUM, the situation is once again transient as the motor speeds up to a higher steady state speed. The switch opens before closing the contact to the next smaller value series capacitor. In this short interval, the applied EMF of the mains is momentarily removed which is driving the current through the motor and creating the magnetic field. The magnetic field starts to collapse thereby inducing an EMF in the winding of the motor which acts to maintain the flow of current L-N. This EMF is in series with the supply EMF: a large voltage across the contacts causes an arc to form, which is drawn as the contacts separate and extinguished. The windings EMF is now effectively connected across the capacitance of the winding to earthed metalwork. The sudden step change in potential difference across this distributed capacitance resulting from the opening of the switch - is now the EMF induced by the collapsing magnetic field, which is much higher than the applied mains (and corresponding back EMF of steady state operation). This step change has frequency components much higher than 50Hz so even though the distributed capacitance my not be large, at these frequencies the capacitive reactance is low - large high frequency current (ie 70mA) flow in the series loop formed by winding, distributed capacitance, earthed frame, CPC, CPC-N connection (Local if TNC-S or remote if TN-S), N back through RCBO and back to winding. There is thus an unbalanced current passing through the RCBO - no similar current in L through RCBO- which it detects and causes it operate.
When the contact eventually makes for MEDIUM, the mains supply is reconnected to the motor windings which causes a further transient, again in voltage across the winding and its distributed capacitance and there is once again current flow through the winding capacitance but now through a series path formed by L, winding, distributed capacitance, earthed frame, CPC, CPC-N connection - no similar current through N of the RCBO this time. Making the contacts also establishes again an L-N loop current through the windings which restores a new steady state at higher speed.
So why does the length of cable between the fan and the RCBO matter? The first thing to realise is that Twin and Earth Cable has different electrical characteristics depending on frequency. All quoted conductances and insulation resistances for example are at DC and low frequencies. At higher frequencies something called skin effect matters; the tendency of the current to flow only in the outer perimeter of the conductor which means at higher frequencies the resistance is higher (conductance is lower). It is why lightning conductors are generally broad tapes of copper (or hollow copper tubes) to create the maximum perimeter area for conductance of the lightning pulse. Thus a longer length of T&E will, at high frequencies, have a higher overall resistance end-to-end.
Next, the way the conductors are arranged in T&E, L-CPC-N, means there is more capacitance between the live conductors and CPC than between the live conductors. A capacitor relies on an insulator between to conducting plates - a dialectric material. A dialectric material is not a perfect insulator hence insulation resistance. The insulation resistance of a long length of T&E is lower than for a short length and dialectrics become weaker as frequency increases thus capacitance decreases.
Earlier I explained how I think in moving from SLOW to MEDIUM SPEED a short pulse of current flows in an N-CPC loop via the RCBO when the switch opens and then another short pulse of current via a L-CPC loop when the switch closes.
These currents are of course caused by short pulses of voltage, the former applied within the N-CPC loop and the latter applied within the L-CPC loop.
For the short length of T&E, there is not as much skin resistance and the impedance between its N conductor and CPC is higher so the pulse of voltage is effectively applied across the N-CPC connection via the N of the RCBO. Along the longer length of T&E there is more skin resistance and lower impedance between the N and CPC which means when the fan is connected only to the longer length of T&E, insufficient is present across the N-CPC connection via the RCBO N path for it to operate - the major part of the current in the loop flows between the conductors of the T&E. Similar reasoning can explain a higher current in the L-CPC loop for the short length of T&E.
A further thought on why the longer length attenuates the pulses of voltage is there are almost certainly more sockets in the longer length of ring with loads connected which will attenuate further the pulses propagating back towards the RCBO when the fan switch is operated SLOW-MEDIUM.
These are my musings on this head-scratcher. Without testing and recording some results for analysis that is all they can be.
A
AndrewMgan1976
Is the cooker hood fitted on a metal stud partition wall? Could be the fixing screw is through one of the studs and is causing a parallel earth
- Reaction score
- 4,325
Midwest, yes, according to my reasoning that is my expectation. I think it is the electrical disturbance/transients caused by the action of switching the cooker hood control (whether up in speed or down) after the motor is run up that precipitates the RCD trip via the short leg of the ring final circuit. It may be though that the transients are of different strengths and forms depending on whether one steps up or down in speed. The only real test is to do it and to to connect up test equipment to analyse why.
All that said, I am conscious that these types of problem are nightmares for the busy electrician and wasteful in time and money. That is why I tried to offer a practical solution first - the filtered socket and maybe transient tolerant RCBO. Be interesting to know if one or other solved the problem.
Rob190's idea of connecting the hood 13A socket further around the ring ( - in the ring or as a spur) was the simplest and cheapest and thus best way to proceed. It could be easily trialled first too.
All that said, I am conscious that these types of problem are nightmares for the busy electrician and wasteful in time and money. That is why I tried to offer a practical solution first - the filtered socket and maybe transient tolerant RCBO. Be interesting to know if one or other solved the problem.
Rob190's idea of connecting the hood 13A socket further around the ring ( - in the ring or as a spur) was the simplest and cheapest and thus best way to proceed. It could be easily trialled first too.
Reply to Cooker hood tripping RCBO - head scratcher in the UK Electrical Forum area at ElectriciansForums.net
Share this page
Similar Threads
Hi,
Just been scratching my head about this one and wonder if I've missed anything.
A few weeks ago I installed a new circuit for a 3kw oven and...
We have a room in the house that was the kitchen. That has been relocated to a different area, so the old kitchen has been sold off and the room...
So was on a call out today and cooker circuit kept tripping the RCD. Disconnected at the board and at the cooker switch and IR’d the cable and...
I will start with apologies, as I have not thought this through very much (at all) yet and have just sat down with a beer.... meaning I thought...
I had a call to a new customer who was experiencing what sounded like nuisance tripping on a kitchen ring.
Some background first.
It's an MK LN...
OFFICIAL SPONSORS
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc
YOUR Unread Posts
-
Is it possible to put a 16mm swa on a 40a mcb protected by an rcd
- Latest: brianmoooore
-
-
Electrical Forum
Welcome to the Electrical Forum at ElectriciansForums.net. The friendliest electrical
forum online. General electrical questions and answers can be found in the electrical forum.
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.