Main Switch twitch......

[NICKYNUNN]

Hi all.
Is there any good reason why you shouldn't use an rcd as a main switch in a domestic installation thats tns/ tncs ?

 

[ImpededLoop]

Section 314 is what you need to look at. The quick answer is that it's not normally acceptable to have an RCD as the main switch in most situations with numerous outgoing circuits.

 

[NICKYNUNN]

Section 314 is what you need to look at. The quick answer is that it's not normally acceptable to have an RCD as the main switch in most situations with numerous outgoing circuits.

Thanks so is it more about all the circuits going off at once, more of a practical/safety reason than electrical reason.

 

[ImpededLoop]

So long as the maximum rated current of the RCD isn't exceeded, then yes.

 

[NICKYNUNN]

So long as the maximum rated current of the RCD isn't exceeded, then yes.

Nice one thanks

 

[ImpededLoop]

Another thing to consider is the DC current from electronic devices which could blind an RCD if several circuits are fed from it.

 

[brianmoooore]

If it's the 'main' CU for a installation of several circuits, then the answer is no, for the reasons given above, but for things like a 2W or 3W CU added on for a couple of extra circuits, such as a shower, or for a small garage CU, it's perfectly acceptable.

 

[pc1966]

Is there any good reason why you shouldn't use an rcd as a main switch in a domestic installation thats tns/ tncs ?

To me there are two separate questions here:

• Can a RCD act as the main isolation switch?
• Is it acceptable to have one RCD for 30mA "additional protection" of all circuits?

For the 1st point, then yes the usual double-pole RCD can be a main switch providing it is rated to at least the maximum system current. So on a 60A fuse the RCD should be 63A or more, etc. Sometimes you will find the sum of MCBs is lower than the supply fuse, so that is another means for limiting total load to the RCD's rating.

For the 2nd point then I would say no, it is poor practice as everything goes off on any trip and as that includes all lights, a safety risk as well. Additionally the accumulated leakage of a whole installation makes spurious trips far more likely to happen.

TL;DR Fine for a 100mA or 300mA delay incomer (as used for TT or farm/fire risk, even if supply is TN) where you have separate groups of 30mA protection, poor for general use except for a very small installation.

 

[NICKYNUNN]

To me there are two separate questions here:

• Can a RCD act as the main isolation switch?
• Is it acceptable to have one RCD for 30mA "additional protection" of all circuits?

For the 1st point, then yes the usual double-pole RCD can be a main switch providing it is rated to at least the maximum system current. So on a 60A fuse the RCD should be 63A or more, etc. Sometimes you will find the sum of MCBs is lower than the supply fuse, so that is another means for limiting total load to the RCD's rating.

For the 2nd point then I would say no, it is poor practice as everything goes off on any trip and as that includes all lights, a safety risk as well. Additionally the accumulated leakage of a whole installation makes spurious trips far more likely to happen.

TL;DR Fine for a 100mA or 300mA delay incomer (as used for TT or farm/fire risk, even if supply is TN) where you have separate groups of 30mA protection, poor for general use except for a very small installation.

nice one

 

[NICKYNUNN]

If it's the 'main' CU for a installation of several circuits, then the answer is no, for the reasons given above, but for things like a 2W or 3W CU added on for a couple of extra circuits, such as a shower, or for a small garage CU, it's perfectly acceptable.

thanks

 

[NICKYNUNN]

Another thing to consider is the DC current from electronic devices which could blind an RCD if several circuits are fed from it.

In this case, I guess you should use a type A rcd or type F , rather than the more common type AC.

 

[ImpededLoop]

In this case, I guess you should use a type A rcd or type F , rather than the more common type AC.

Yes indeed. Type AC are not really suitable now due to electronic loads. Rcbo's are the usual way to go now. What's the application that you're thinking of using an upfront RCD for?

 

[pc1966]

In this case, I guess you should use a type A rcd or type F , rather than the more common type AC.

Realistically type AC ought not to be made any more (or even for last few years) as so many products have some capacity to leak an element of DC.

As @ImpededLoop says most RCBO (at least all I have seen in recent years) are type A which ought to be your base line for RCD/RCBO choices.

 

[timhoward]

There are excellent answers above which I agree with.
It's quite common (in my area at least) to find a semi-rural row of 2 up 2 down terraces with overhead supply, TT earthing, and a fairly old 5 way consumer unit with an RCD main switch.
My additional comment would be that the chances of pressing the test button are (even) lower than the usual 0% if you lose all power at once by doing so, and RCD's need occasional exercise to continue doing the job they are designed to do. To my thinking there is an increased risk of the RCD not working when it's the main switch as it's probably sat there untested most of it's life.

 

[pc1966]

RCD's need occasional exercise to continue doing the job they are designed to do.

Agreed! If never tested you have no idea if it would detect a fault and even if it does, would the mechanism actually trip OK.

To my thinking there is an increased risk of the RCD not working when it's the main switch as it's probably sat there untested most of it's life.

If I were installing a CU for a TT system it would be a 100mA delay RCD as incomer and RCBOs for the individual circuits. Sure the 30mA protection might have failed so you don't have proper shock protection, but a 100mA delay is better than nothing and it prevents the CPC/earth rod sitting at 230V as would otherwise happen if the only RCD failed to trip So whole-house ADS is far better than none at all!