Selectivity at Static Caravan

[JoeMagin]

Hi all,

Just a electrical student here, and on holiday at a caravan park so, of course, i go snooping about a bit for the electrics.

I notice the caravan at the hook up is fed off a 30mA RCD and 32A MCB. The board within the caravan contains a 30mA RCD and a 32A ring.

As im just learning I might be being completely green here, but in terms on discr.. ahem... selectivity, wouldnt the RCD and the MCB at the hook up have to be rated higher.

Are caravan sites special cases for tbese sorts of things? Any clearning up would really scratch that itch at the back of my mind.

Cheers!

 

[plugsandsparks]

Hope you having a great holiday ! - No sweat over selectivity. Caravan Hook up providers have to allow for worst case (Caravan with very rudimentary electrics) - You are right though if there is a fault, likely to be Hook up RCD first or both together.

 

[buzzlightyear]

green here, but in terms on discr.. ahem... selectivity, wouldnt the RCD and the MCB at the hook up have to be rated higher. Time

No, because the rcd in the caravan is lower then the hook up, down stream is correct selectively 63amp time delay is I order before it takes out the main cut out. Or other. I would be more bothered the caravan not up to date on testing and the hook up, eye spy.

 

[pc1966]

It is unusual to see 32A for the supply, many hook-up points are fed with 6-16A MCBs (often C-curve for slightly better handling of surges). The issue of selectivity applies more for cases when:

• The upstream device (RCD/MCB/fuse) supplies multiple circuits, or
• The protective device(s) are locked against unauthorised access to reset them

Then any fault on a final load has to potential to cause significantly more and long-lasting outages.

In the caravan situation normally the folks using the caravan can easily reset both RCDs so the impact of one or both tripping is less serious. However, the impact of a failed/stuck RCD and the higher that usual risk from a metal vehicle with cables flexed many times means it is sensible to have two RCDs, both with 30mA "instant" protection against shock, providing no single point of failure as the most safe and sensible approach.

Generally speaking MCBs in series have poor to very-poor selectivity and ought to be avoided from a design point of view, but again there are cases when the risk of greater outage is less than the cost/complexity of a better solution such as an MCCB or fuse upstream of the MCB. That is either very expensive (MCCB) or unsuitable for most electrically-unskilled folks to fix (fuse, especially bolted-in ones).

 

[nicebutdim]

In the caravan situation normally the folks using the caravan can easily reset both RCDs so the impact of one or both tripping is less serious. However, the impact of a failed/stuck RCD and the higher that usual risk from a metal vehicle with cables flexed many times means it is sensible to have two RCDs, both with 30mA "instant" protection against shock, providing no single point of failure as the most safe and sensible approach.

Upstream RCD has 0.1s printed on it and appears to be 30mA time delayed.

 

[mainline]

Upstream RCD has 0.1s printed on it and appears to be 30mA time delayed.

It's not time delayed, the 0.1s is the fault current cut off time.

 

[pc1966]

Upstream RCD has 0.1s printed on it and appears to be 30mA time delayed.

I didn't think they made time-delay 30mA devices, and in any case 0.1s is within the test limits for In for "instant" RCDs.

 

[mainline]

I didn't think they made time-delay 30mA devices, and in any case 0.1s is within the test limits for In for "instant" RCDs.

https://www.electricalcounter.co.uk/products/Circuit+Protection/Cudis+CPN+Circuit+Protection/Devices/Time+Delayed+RCDs/Cudis+CPN+RC263-030T+63A+Double+Pole+Type+AC+30MA+Time+Delayed+RCD/545305925

 

[pc1966]

Interesting, I guess it makes sense if you had some 10mA final circuits downstream (RCD sockets, etc), etc, but odd as usually 30mA have to be fast enough for "additional protection".

 

[mainline]

Interesting, I guess it makes sense if you had some 10mA final circuits, etc, but odd as usually 30mA have to be fast enough for "additional protection".

Probably Chinese non time delayed ones that failed on the production line