neutral to earth fault?
- Thread starter kevin1234
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We are not permitted to fuse the neutral supply. We can use twin pole MCB's but this is rare in domestic. So in theory with a twin pole MCB you could get it to trip with earth neutral fault but in practice no it will never have enough current to trip it.
The RCD or RCBO may trip with earth neutral fault this will depend on earthing system and with TN-C-S length of cable to where it is combined. Normally it will trip with RCD but you could not guarantee it will trip.
The RCD or RCBO may trip with earth neutral fault this will depend on earthing system and with TN-C-S length of cable to where it is combined. Normally it will trip with RCD but you could not guarantee it will trip.
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Kevin, with respect, we went through this with you six months ago.
//www.diynot.com/forums/viewtopic.php?t=200941
Please tell me you understand the difference between an MCB and a RCD now?
//www.diynot.com/forums/viewtopic.php?t=200941
Please tell me you understand the difference between an MCB and a RCD now?
I really think that he should look for another career - I don't think he's cut out to be an electrician.
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I will be kind and assume this is something being done in a college and they are working out all the options.
One of the problems is current flow to earth is very dependent on the earthing system. With a system using the ground as a return for earth (TT) the voltage differential between earth and neutral can be high so very unlikely any RCD would hold in with such a fault but the current is low so also unlikely that even with twin pole MCB would there be enough current to trip within the time required.
Move to other extreme where the earth and neutral are bonded together for a good proportion of the run from step down transformer (TN-C-S) then the voltage differential can be very low in fact if no load it could be zero so a neutral earth fault could completely fail to trip a RCD.
As the load is increased so the voltage differential will increase and then we have to start doing sums. So with the scenario of an earth neutral connection of 1 ohm and a resistance between the RCD and split point of neutral and earth of 0.1 ohm that would mean with a load of 650 ohm you would still not trip a RCD. Simulation with Multisim shows 29ma neutral to earth flow.
Move the figures around a bit and have a 10 ohm earth neutral connection and the appliance load could be 80 ohms. This does happen and often people blame the wrong appliance for tripping the RCD as a result. It also explains why a load not going through the RCD can cause it to trip as it raises the voltage differential between earth and neutral.
In late 80’s and early 90’s when we moved to using RCD protection many problems were caused by single pole switching and an appliance with a resistance of a few hundred ohms between earth and neutral could be a nightmare to find and as load increased it would cause system to trip. After the RCD firms started to use linked switches and MCB’s and started to switch the neutral, which until then had been considered as bad practice.
Go to the continent where RCD’s have been used for longer than here and many consumer units use twin pole MCB’s making fault finding so much easier. There is of course a second reason for using twin pole and that’s the use on TT supplies and reversible plugs and sockets and they truly regard the neutral as a “Live”. Here only in the last few years has the term “Line” been used to refer to the Phase wire, it was historically called the “Live”.
We now have some odd RCD’s/RCBO’s which only switch the “Line” and leave the neutral connected mainly to allow them to be used in consumer units originally designed for single pole MCB’s and to upgrade them to RCBO’s. If one looks at firms like Gewiss who do not sell much in UK but mainly supply Europe you will find it hard to find any single pole RCD’s/RCBO’s
One of the problems is current flow to earth is very dependent on the earthing system. With a system using the ground as a return for earth (TT) the voltage differential between earth and neutral can be high so very unlikely any RCD would hold in with such a fault but the current is low so also unlikely that even with twin pole MCB would there be enough current to trip within the time required.
Move to other extreme where the earth and neutral are bonded together for a good proportion of the run from step down transformer (TN-C-S) then the voltage differential can be very low in fact if no load it could be zero so a neutral earth fault could completely fail to trip a RCD.
As the load is increased so the voltage differential will increase and then we have to start doing sums. So with the scenario of an earth neutral connection of 1 ohm and a resistance between the RCD and split point of neutral and earth of 0.1 ohm that would mean with a load of 650 ohm you would still not trip a RCD. Simulation with Multisim shows 29ma neutral to earth flow.
Move the figures around a bit and have a 10 ohm earth neutral connection and the appliance load could be 80 ohms. This does happen and often people blame the wrong appliance for tripping the RCD as a result. It also explains why a load not going through the RCD can cause it to trip as it raises the voltage differential between earth and neutral.
In late 80’s and early 90’s when we moved to using RCD protection many problems were caused by single pole switching and an appliance with a resistance of a few hundred ohms between earth and neutral could be a nightmare to find and as load increased it would cause system to trip. After the RCD firms started to use linked switches and MCB’s and started to switch the neutral, which until then had been considered as bad practice.
Go to the continent where RCD’s have been used for longer than here and many consumer units use twin pole MCB’s making fault finding so much easier. There is of course a second reason for using twin pole and that’s the use on TT supplies and reversible plugs and sockets and they truly regard the neutral as a “Live”. Here only in the last few years has the term “Line” been used to refer to the Phase wire, it was historically called the “Live”.
We now have some odd RCD’s/RCBO’s which only switch the “Line” and leave the neutral connected mainly to allow them to be used in consumer units originally designed for single pole MCB’s and to upgrade them to RCBO’s. If one looks at firms like Gewiss who do not sell much in UK but mainly supply Europe you will find it hard to find any single pole RCD’s/RCBO’s
Given that 5 months ago we explained to him what RCDs and MCBs do, and here he is asking the same question I honestly think it would be kinder to encourage him to change direction before he loses any more of his life to something that's not for him.
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