Caravan Hook-Up RCBO Current Rating

[ArchieB68]

I have a small caravan park with 8 pitches. Each have a Rolec Electrical Hook-Up unit fitted with a Rolec 16A Single Pole and Neutral 2 Module C Curve 6kA 30mA RCBO.

I've recently fitted meters to these units which display voltage, Current and kW. I was surprised to see that some of the caravans were displaying currents over 20A and the 16A RCBO still had not tripped. I checked this myself with a 5kW load on all 8 units and not one of the 16A RCBO's tripped.

Should these RCBO's not trip if the current goes over the rated16A?

 

[happysteve]

The RCBO is operating correctly, according to its standard (EN 61009). Looking at the overcurrent time/current characteristics, a 16A device will never trip at 20A. It will trip at 25A after (very approximately) 15 minutes or so. 16A is not a hard limit.

 

[pc1966]

As above, the MCB is designed to have a thermal overload protection curve to match a cable, so a small overload for a long time, or a large overload for a short time, is needed to trip it.

Then they have a 'fault' trip where the magnetic part kicks in and for a C-curve MCB that is between 5 and 10 times its rating (so your 16A C device would go in tens of milliseconds or less at 80-160A fault).

Finally the RCD side of the RCBO is designed to trip on an imbalance of current that is between 0.5 and 1 times its rating, so the typical 30mA sort used for most final circuits will not trip below 15mA, might trip for 15-30mA and should always trip above 30mA.

 

[littlespark]

The age old question…. What are people doing in their caravans to generate anything near 16A?

We had statics where the owners had about 6 or 7 different electric heaters, and wondered why the electric tripped.
My answer was always….. “why have you got electric heaters in a gas powered centrally heated van?”

 

[ArchieB68]

Thanks. That explains for the extra power consumption I wasn't expecting over the winter. Some caravans seem to be drawing more than 16A over 24 hours which meant that with our pitch price, which was inclusive of electricity back then, we were running at a loss!
I've now installed smart meters but they limit the supply to 20A and then shut down. Unfortunately the customers can't reset these themselves so I'm getting calls all day and night. I need the RCBO to trip before the supply hits 20A. Any ideas?

 

[littlespark]

Yes you’ll find that
. Electricity is inclusive…. Ie, they paid for it with pitch fees…. So they’ll use what they can with electric to save the gas that they’ve bought.

 

[pc1966]

Thanks. That explains for the extra power consumption I wasn't expecting over the winter. Some caravans seem to be drawing more than 16A over 24 hours which meant that with our pitch price, which was inclusive of electricity back then, we were running at a loss!

Yes, recently that would be a real penalty.

I've now installed smart meters but they limit the supply to 20A and then shut down. Unfortunately the customers can't reset these themselves so I'm getting calls all day and night. I need the RCBO to trip before the supply hits 20A. Any ideas?

Fit a 10A RCBO C-curve. That will cope with a 2kW heater and light electronics use, or a 3kW / 13A kettle for enough time to boil the water, etc.

If you need lower average but a bit more switch-on surge protection you don't get D-curve RCBO but you can use a D-curve MCB and separate RCD, if there is space for it.

 

[pc1966]

As an aside, the fancy electronic-trip MCCB allow you to adjust the trip curve in many ways to get the characteristics you want, but they are seriously expensive (usually not much change from £1000) and targeted at much larger system.

 

[littlespark]

Running a 16A breaker overload for an extended time period can damage it.

Now the hook ups are metered…. Charge them per unit… then see how much they use. I bet they’ll reduce consumption.

(Check. I don’t think you can charge per unit any higher than you pay from your supplier)

 

[pc1966]

(Check. I don’t think you can charge per unit any higher than you pay from your supplier)

I was wondering about that, and if that applies everywhere or just for a BNO/landlord powering flats, etc?

At my work we are going to have another metal cabin for a client on site and were proposing to charge electricity as 5% above our rate based on the meter for them to allow for the losses in the sub-main, etc, feeding the site as our landlord meters at the busbar chamber end. But that is more a commercial lease sort of deal and maybe very different.

 

[littlespark]

I’m pretty sure the caravan site license didn’t allow us to charge any more… or maybe a “reasonable” amount more considering the infrastructure we needed to put in place regarding meters, and administration costs.

Although we did have a PV array of 200 panels… so customers were paying us for the free power generated.

 

[Lucien Nunes]

I need the RCBO to trip before the supply hits 20A. Any ideas?

RCBOs and MCBs are not designed to be used for limiting consumption by users. They are designed to protect cables, and their characteristics match those of a cable. There is a ratio called the fusing factor, 1.45 for a standard MCB, that gives the lowest must-trip current for a given rated current. This is fine for cables, as the current they can withstand is significantly higher than the 'ideal' current that would keep them cool enough to last their maximum theoretical lifespan. An MCB with a lower fusing factor would not significantly improve cable safety but would be more prone to nuisance tripping, so this figure is chosen as a good compromise for general use.

To limit the sustained load to less than 1.45 x 16A using an MCB one would need a device rated at less than 16A, but then to deliberately run that at 16A would be poor design. One should not, ideally, design for a load between In and 1.45In, as this causes the MCB to run hot and possibly have a shortened life itself. The 'correct' method of closely limiting the load on a 16A circuit is to use a 16A MCB / RCBO for circuit protection, but a separate load-sensing relay for 'functional' load limiting. This could have any threshold and time characteristic set, according to how many nuisance trips you can accept etc. Load-sensing relays are expensive and probably unrealistic for a caravan pitch hookup, but technically they are the correct method for monitoring and limiting greedy loads, not relying on the circuit protection to do it.

 

[pc1966]

To limit the sustained load to less than 1.45 x 16A using an MCB one would need a device rated at less than 16A, but then to deliberately run that at 16A would be poor design. One should not, ideally, design for a load between In and 1.45In, as this causes the MCB to run hot and possibly have a shortened life itself.

Which is very true. But in practical terms it is cheaper to replace a couple of 10A RCBO over a decade than to implement a fancy user-re-settable load shedding arrangement

 

[happysteve]

Similar to pc1966's suggestion of lowering the RCBO rating... leave the RCBOs as they are, but assume folks will pull 20A and charge for your pitches accordingly so it reflects the real-life consumption and you don't lose out.

EDIT: oops, sorry, have just read the later post by the original poster, I see he needs to limit supply to below 20A. Ignore the above!

 

[brianmoooore]

A touring caravan is designed for a maximum load of 16A, so a 16A MCB/RCBO at the supply socket is within design limits.
Unfortunately, the occupants of the caravan (and I'm as guilty as the rest), will always push the loading to whatever they can get away with without losing supply.
10A type C MCB and a separate 25A 30mA RCD is my preferred solution. MCB may need replacing occasionally, but at least the more expensive RCD won't be overloaded and damaged.