Another question on spurs

[terryb630]

I need to power a summer house, I need to fit at least 2 double sockets from one of which I may have to run a heater, the other will have to power lights and heater lights to create a tortoise habitat for the winter months.
Can I spur from the house ring via a RCD to create what is effectively a small ring like this? I would have preferred to run a cable from the CSU but it's physically impossible.
many thanks in advance
Terry B

 

[ban-all-sheds]

You can't have a ring in the summerhouse, and you need an FCU at the house end of the spur, not an RCD. Although you will need an RCD FCU if the house circuit is not already RCD protected. Make sure the FCU is double pole so that you can isolate the outside stuff if you need to.

You'll be limited to 13A in the summerhouse.

You'll need to consider voltage drop, fault loop impedance, testing, earthing systems and suitable installation techniques for outdoor cables.

• 
  
  
  
• http://www.diynot.com/wiki/electrics:books
  
  
• http://www.diynot.com/wiki/electrics:mcbtocable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm (all subsections)
  
  
• http://www.tlc-direct.co.uk/Book/4.3.11.htm
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  
  
• http://www.tlc-direct.co.uk/Book/5.3.1.htm
  
  
• http://www.diynot.com/wiki/electrics:mcb2
  
  
• http://www.tlc-direct.co.uk/Book/7.13.3.htm
  
  
• http://www.tlc-direct.co.uk/Book/8.1.1.htm
  
  
• http://www.diynot.com/wiki/electrics:swa-gland
  
  
• http://electrical.theiet.org/wiring-matters/16/elect-inst-outdoors.cfm?type=pdf
  
  
• http://www.diynot.com/wiki/electrics:part-p
  

 

[ericmark]

I am sure you can do what ever you like but would guess your asking about either safety or regulations. It is near impossible for DIY people to follow regulations so I will only refer to safety.

Using a RCD FCU or if RCD already provided a FCU (fused connection unit) and cable 2.5mm or more if you get it wrong likely it will fail safe.

At 13A there is no real need for a ring. Conductors in parallel may reduce volt drop and improve the loop impedance but not really what we would call a ring.

 

[mfarrow]

For safety & convenience I would try and put the RCD in the summer house, assuming you'll be running an armoured cable to it from the house.

 

[ericmark]

For safety & convenience I would try and put the RCD in the summer house, assuming you'll be running an armoured cable to it from the house.

Not sure on that. In winter months one may not want to leave the house to reset a RCD would be easier to reset in the house.

Also in the out building likely you will need an active RCD as likely you have no way to measure the loop impedance to insure volt drop will not prevent it tripping under fault conditions. However in the house less of a problem with volt drop so a passive type could be used.

Also you could rotavate the cable in error one day and in the house still protected by the RCD.

 

[JohnW2]

Also in the out building likely you will need an active RCD as likely you have no way to measure the loop impedance to insure volt drop will not prevent it tripping under fault conditions. However in the house less of a problem with volt drop so a passive type could be used.

Could you perhaps help me to understand that, because none of it really makes any immediate sense to me. Thanks.

Kind Regards, John

 

[ericmark]

Also in the out building likely you will need an active RCD as likely you have no way to measure the loop impedance to insure volt drop will not prevent it tripping under fault conditions. However in the house less of a problem with volt drop so a passive type could be used.

Could you perhaps help me to understand that, because none of it really makes any immediate sense to me. Thanks.

Kind Regards, John

The RCD has electronic components within it which require voltage to operate. Where a fuse or MCB has been selected with regard to the earth loop impedance then with a massive overload the MCB or fuse will open before the voltage has dropped below the point where the RCD will fail to work. However most DIY people do not measure the loop impedance and under fault conditions the voltage may fall below the point where the device will work.

With an active RCD there is no problem as once the voltage drops below the thresh hold level it will open anyway so it fails safe. However with a passive RCD this is not the case and they do not fail safe.

An error with some 110 volt installations first alerted me to the problem the loop impedance of the transformer was too high for either the MCB or the RCD to open when the long cable used was short circuited and the cable melted and caused a fire. I would agree with 230 volt which is not centre tapped like the 110 volt supply it is unlikely that this would happen however with the lack of testing done with DIY there is always a chance.

So where using passive RCD's one should strive to get as close to the supply as one can and still protect the individual circuit where testing is not carried out. One would assume the ring was tested on installation so fitting a passive RCD FCU directly to the ring should not be a problem and it would mean if there is a power cut the device would not need resetting.

Where the power in the out building is used by the occupants having an active RCD is not a problem they are on hand to reset it. But where devices are left unattended and their failure could have unwanted effects be it a freezer or in this case tortoise heater then having an active RCD is not really a good idea.

I have been talking about the whole problem where we know the DIY guy will not inspect and test as laid down by the regulations and if we accept that they will not do the testing then to have a reasonable safety margin then there are things which can be done to reduce the risk which would not apply when the same job was being done by an electrician who would have course have done all the calculations inspecting and testing.

Ask the standard DIY guy what the ELI was and he will give you a blank look and try to find some one who talks his language. Yes two wrongs don't make a right and the DIY guy should not play with things he does not understand and all that but we all know he will. So we need to at least try to use language he will understand and a method which will provide at least some protection even if he makes mistakes.

I will of course make errors. I for example failed to explain that an active RCD will open with a power failure and a passive RCD will not. Also that FCU means fused connection unit. Maybe we should also say a RCD is a residual-current device but would that mean anything anyway. To the DIY guy it's a device which trips when current goes to earth. We may think of it as a device which trips when there is an imbalance but that's not talking DIY language.

 

[bernardgreen]

The RCD has electronic components within it which require voltage to operate.

Are you sure ? The RCDs I have opened have had nothing more than the toroid current transformer and a soleniod to trip the mechanism.

The mechanism was "armed" by pressing the lever down and then up to close the contacts. The arming stores mechanical energy in a spring. The soleniod when energised by a output from the toroid releases this stored mechanical energy to force open the contacts.

That I consider to be a passive RCD,

I have seen RCDs with electronic amplifiers but these were those that tripped at 10 mA differential where a toroid with one turn could not produce enough energy to reliably operate the solenoid. That I consider to be an active RCD.

I have seen RCDs with several turns of Live and Neutral in the toroid which, without electronic amplification, provided enough energy to operate the solenoid with very low differentials ( very low earth leakage ).


I cannot understand the "advantage" of using electronics to trip the mechanism when loss of neutral would render the RCD useless as without power to the electronics the mechanism cannot be tripped even if there is a leak from the still connected Live to ground. The "functional" earth in RCBOs may be the solution to a loss of incoming neutral.

I suspect a passive ( no electronics ) RCBO is possible but would be a complex mechanism with three trigger mechanisms and thus expensive to manufacture. ( thermal over current, magnetic over current and differential )

 

[JohnW2]

Could you perhaps help me to understand that, because none of it really makes any immediate sense to me. Thanks.

The RCD has electronic components within it which require voltage to operate. Where a fuse or MCB has been selected with regard to the earth loop impedance then with a massive overload the MCB or fuse will open before the voltage has dropped below the point where the RCD will fail to work. However most DIY people do not measure the loop impedance and under fault conditions the voltage may fall below the point where the device will work. ... With an active RCD there is no problem as once the voltage drops below the thresh hold level it will open anyway so it fails safe. However with a passive RCD this is not the case and they do not fail safe.

An interesting suggestion, but I see several problems with it (even if there are electronics in the RCD).

Firstly, the what you say here seems to be back-to-front in terms of the argument you were presenting. The potential difference between L & N of the RCD (which is the voltage that will drive any electronics in the RCD) will surely be at it's lowest when a fault (L-E or L-N) is closest to the RCD. If the component of the fault loop impedance due to the final circuit (i.e. {R1+R2} or {R1+Rn}) between RCD and fault is high, then that will result in the L-N pd at the RCD (which powers any electronics) being higher, wouldn't it? This seems to be the opposite of what you were suggesting. Furthermore, even with an active RCD, I would have thought that the L-N voltage across it would be more likley to fall to the threshold for it disconnecting if the fault were very close to the RCD.

Secondly, if a very low impedance fault suddenly arises, the L-N pd at the RCD will immediately fall to a very low level, so I don't see how a rapidly operating MCB or fuse (downstream of the MCB) could operate "...before the voltage has dropped below the point where the RCD will fail to work."

Thirdly, if what you say were correct, then would that not mean that passive RCDs would not offer satisfactory protection against low impedance L-E faults in a TT installation, even though one is then reliant on an RCD to provide that protection (since there is almost no hope of an MCB/fuse operating)?

Kind Regards, John

 

[JohnW2]

The RCD has electronic components within it which require voltage to operate.

Are you sure ? The RCDs I have opened have had nothing more than the toroid current transformer and a soleniod to trip the mechanism. The mechanism was "armed" by pressing the lever down and then up to close the contacts. The arming stores mechanical energy in a spring. The soleniod when energised by a output from the toroid releases this stored mechanical energy to force open the contacts. ... That I consider to be a passive RCD, ... I cannot understand the "advantage" of using electronics to trip the mechanism when loss of neutral would render the RCD useless as without power to the electronics the mechanism cannot be tripped even if there is a leak from the still connected Live to ground. The "functional" earth in RCBOs may be the solution to a loss of incoming neutral.

That's what I used to think. However, as you will probably recall, when I recently opened my daughter's blown-up RCD, I found:

Untitled

• JohnW2
• 20 Apr 2013
• 2

The 1.8kΩ resistor in the foreground appears to be the 'test resistor', but I don't know what the rest does - presumably something to do with control/operation of the RCD. I'll have a closer look and see if I can work out what that electronics is all about.

Kind Regards, John

 

[JohnW2]

That's what I used to think. However, as you will probably recall, when I recently opened my daughter's blown-up RCD, I found:

Untitled

• JohnW2
• 20 Apr 2013
• 2

The 1.8kΩ resistor in the foreground appears to be the 'test resistor', but I don't know what the rest does - presumably something to do with control/operation of the RCD. I'll have a closer look and see if I can work out what that electronics is all about.

I've just had a look, and it's definitely 'control electronics'. There is a bridge rectifier to provide the power and the active component is an (8-pin SIL) VG54123 IC, which is a 'Ground Fault Interrupter'.

I have to say that, like Bernard, I'm a little surprised. Of course, what is not clear without a better understanding of the electronic functionality (or experimentation!) is the extent to which the electronics are crucial to tripping in the presence of L-N imbalance (although I suspect that it is crucial!). I will investigate further! There is a modest (1μF) capacitor across the power supply to the IC - which may be enough to keep it powered for long enough for the RCD to trip, even in the absence of an L-N pd across the RCD terminals.

Kind Regards, John

 

[EFLImpudence]

The RCD has electronic components within it which require voltage to operate. Where a fuse or MCB has been selected with regard to the earth loop impedance then with a massive overload the MCB or fuse will open before the voltage has dropped below the point where the RCD will fail to work.

I am struggling to understand why that matters or

why the RCD would have tripped anyway.

 

[JohnW2]

I've just had a look, and it's definitely 'control electronics'. There is a bridge rectifier to provide the power and the active component is an (8-pin SIL) VG54123 IC, which is a 'Ground Fault Interrupter'... I will investigate further!

Typical appllication of VG54123:

Untitled

• JohnW2
• 22 Jun 2013
• 1

and Operating Characteristics:

Untitled

• JohnW2
• 22 Jun 2013
• 1

It's simply amplifying the signal from the sense coil and then using an SCR to trigger the trip solenoid. There's no way it will work without the elctronics working. However, with a typical operating current of only 0.4mA, maybe it will carry on working for long enough from the capacitor (2.2μF in the Application Note), even if L-N pd disappears.
Edit: on reflection, even if the capacitor did keep the electronics working 'for long enough', the SCR would not be able to operate the trip solenoid if L-N pd had fallen to near zero - so I remain a bit confused.
Edit 2: ... unless, of course, it's a 'normally activated' solenoid!

Kind Regards, John

 

[JohnW2]

The RCD has electronic components within it which require voltage to operate. Where a fuse or MCB has been selected with regard to the earth loop impedance then with a massive overload the MCB or fuse will open before the voltage has dropped below the point where the RCD will fail to work.

I am struggling to understand why that matters or ... why the RCD would have tripped anyway.

I presume he's talking about a very low impedance L-E fault, and is contemplating a situation in which the MCB/fuse did not operate. As I've demonstrated, in at least one RCD (my daughter's) power to its electronics is crucial to the RCD operating, so if that voltage did disappear, it would not operate.

Kind Regards, John

 

[bernardgreen]

I can see the SCR that fires and the contacts that open circuit the Live and Neutral but I cannot see the symbol for a coil or other means of operating the contacts.