Get power to gazebo for lighting

[Harry Bloomfield]

I guess the only additional reason would be that it trips faster than the one on the house CU... For convenience... But perhaps no safer

It will not necessarily trip any faster, it would be a toss up, as to which tripped, or both could trip.

 

[JohnW2]

Which means a fault in the gazebo could suddenly and without warning plunge the house into darkness. A hazard to people in the house. .... Feed the cable to the gazebo from its own RCD and MCB ( or an RCBO ) in the CU in the house to avoid that hazard,

That's obviously true - but IF, for whatever reason, the OP wants to continue feeding the gazebo from a circuit which is protected by a 'house RCD', there would then really be no significant downside of having the second RCD he proposes. In fact, I would have expected that you would have been one of the last people to criticise someone who wanted redundancy of protective devices?

Kind Regards, John

 

[bernardgreen]

redundancy of protective devices

is good, but when one of them can unnecessarily cut the power to an entire house instead of just the gazebo then the use of two may not be sensible.

It goes without saying that to have an RCBO in the house to protect the cable to the gazebo ( and nothing else ) with an RCD in the gazebo would provide the redundancy of protection.

 

[JohnW2]

is good, but when one of them can unnecessarily cut the power to an entire house instead of just the gazebo then the use of two may not be sensible.

Sure - but, as I said, I was talking specifically about the situation in which, for whatever reason, it had been decided to feed the gazebo (with or without an additional RCD) from an RCD in the 'house CU'.

In that situation, there is virtually no downside to an 'unnecessary' downstream additional RCD. On the contrary, there is at least a chance that the downstream one, but not the upstream one, would trip in the case of a fault (thereby not 'plunging the house into darkness). If, as is quite likely, both were to trip, that's not significantly different from reliance on just the 'house' RCD, the only difference being the need to also reset the downstream one.

It goes without saying that to have an RCBO in the house to protect the cable to the gazebo ( and nothing else ) with an RCD in the gazebo would provide the redundancy of protection.

As you say, that goes without saying - but that is a scenario that we haven't even considered.

Kind Regards, John

 

[Harry Bloomfield]

In that situation, there is virtually no downside to an 'unnecessary' downstream additional RCD. On the contrary, there is at least a chance that the downstream one, but not the upstream one, would trip in the case of a fault (thereby not 'plunging the house into darkness). If, as is quite likely, both were to trip, that's not significantly different from reliance on just the 'house' RCD, the only difference being the need to also reset the downstream one.

Having been in this situation, the usual scenario is that one RCD proves to be a little more sensitive than the other, so that one will always be the first to respond, first to disconnect.

 

[JohnW2]

Having been in this situation, the usual scenario is that one RCD proves to be a little more sensitive than the other ...

At some level of precision, that is essentially inevitable since the probability of two devices having absolutely identical thresholds, to the nearest picoamp/picosecond or whatever, is close to zero. However ....

, so that one will always be the first to respond, first to disconnect.

... because 'moving parts' are involved, there is a finite time between when the device is 'triggered' and when its contacts open. That means that there is a time-window (of continuing current flow) during which the second one may also be triggered.

I probably agree that the most common situation is probably for one RCD to operate, but the other not. However, it is far from unknown (at least, in my experience) for both to operate.

Kind Regards, John

 

[Harry Bloomfield]

I probably agree that the most common situation is probably for one RCD to operate, but the other not. However, it is far from unknown (at least, in my experience) for both to operate.

I would surmise, that under a low but increasing level of current leakage, the usual fault condition, that the same RCD will always be the one to trip - the more sensitive one. Under a much larger current fault condition, then it becomes a race, between the two, the first to disconnect, or more likely both disconnecting.

 

[JohnW2]

I would surmise, that under a low but increasing level of current leakage, the usual fault condition, that the same RCD will always be the one to trip - the more sensitive one. Under a much larger current fault condition, then it becomes a race, between the two, the first to disconnect, or more likely both disconnecting.

I would agree .... I'm not so sure that the latter situation is as rare as you seem to imply. The residual current does not need to be 'much larger' for that to be the case - merely larger than the trip threshold of the less sensitive RCD.

Kind Regards, John

 

[JohnW2]

Indeed, but you still don’t need that second RCD.

The degree of "need" is in the eye of the beholder. As above, some people derive reassurance from redundancy of protective devices - not all that unreasonable in this case, given that failure (or failure to remain in-spec) of RCDs is far from unknown.

It's also worth pointing out that, if the house has RCDs, then the risk of at least part of the house being plunged into darkness as a result of a fault is the same if the fault arises within the house as if it arises outdoor. Whilst the latter is undoubtedly more common (because of the increased likelihood of water ingress), it can also happen due to an 'indoor fault'.

As I always say to those people who are very concerned about the potential (very small) risk ('to life and limb') which can result from being 'plunged into darkness', no number of RCDs or RCBOs will prevent that happening in the event of a power cut or other cause of widespread failure within the installation - so people with such concerns really should address them by installing 'emergency lighting'.

Kind Regards, John

 

[tractor-pete]

So... The festoon lights I'm looking at are a professional range running on 240V with a rectifier which converts to 240V (!!! According to supplier).

I contacted the supplier and they said I need an RCD Type B... Rather than the standard Type AC.

What type of RCD is usually in the consumer unit of a normal house?

Equally I can't find any outdoor RCD of type B.

Going round in circles now haha

 

[JohnW2]

I contacted the supplier and they said I need an RCD Type B... Rather than the standard Type AC.

They may or may not be right.

What type of RCD is usually in the consumer unit of a normal house?

In the Uk, traditionally Type AC but, now, increasingly Type A. I think it would be pretty (very?) unusual to find a Type B.

Equally I can't find any outdoor RCD of type B.

They may well not exist (although I dare say that someone will pop up to demonstrate that they do )

Going round in circles now haha

Whilst I can't argue about the theory, I suspect that is because you are probably being over-concerned/over-cautious (in comparison with most other people). I would strongly suspect that the vast majority of users of these lights would never even consider thee sort of issues you are raising.

Kind Regards, John

 

[tractor-pete]

I take your point, but I think these are semi-professional/commercial lights and are expected to run up to 1000m which is why they run LEDs on 240V DC.

So the average dosmetic user wouldn't realise the difference

I was hoping a sparky would set me right

 

[JohnW2]

I take your point, but I think these are semi-professional/commercial lights and are expected to run up to 1000m which is why they run LEDs on 240V DC.

The possibly very long run might explain the high voltage (to reduce current, hence voltage drop) but all LEDs necessarily ultimately work on DC, regardless of the voltage that is being distributed, so I would have thought that any LED lights can, at least theoretically, result in DC leakage currents (which are what an RCD seeks to detect) in the event of a fault..

So the average dosmetic user wouldn't realise the difference

See above. I'm not convinced that the situation is necessarily any different in sets of LED lights intended for domestic use/users.

Maybe someone else can think more clearly about this than I am managing

Kind Regards, John