High Zs on a Campsite

[Sparky Col]

I have been asked to provide a quote for a supply to an toilet block on a campsite.
The three phase supply to the campsite is approx. 150m from the mains incoming supply and fed off an 40amp type b mcb. The cable is 35mm 5-core SWA.
The issue I have is when I measured the Zs at the campsite DB it is 0.86 ohms, which is very much on the edge.
I don't want to add any additional circuits because of this.
What are my options to get a better Zs ?
An 30mA rcd at origin would cause issues as there is sub boards, can you earth spike at the campsite DB in addition to the supply earth ? or should I just make every circuit off the campsite an TT supply and earth spike each one ?

 

[MDJ]

I have been asked to provide a quote for a supply to an toilet block on a campsite.
The three phase supply to the campsite is approx. 150m from the mains incoming supply and fed off an 40amp type b mcb. The cable is 35mm 5-core SWA.
The issue I have is when I measured the Zs at the campsite DB it is 0.86 ohms, which is very much on the edge.
I don't want to add any additional circuits because of this.
What are my options to get a better Zs ?
An 30mA rcd at origin would cause issues as there is sub boards, can you earth spike at the campsite DB in addition to the supply earth ? or should I just make every circuit off the campsite an TT supply and earth spike each one ?

so the new block is 150 mtrs from the mains supply? well why cannot you rod the new block and make it TT from the new board in the shower block with every circuit on a seperate RCBO and have just a double pole main switch for the DB to avoid nusience tripping, have I explained that properly? I hope so, treat the block as an independent building with its own earthing arrangement due to the distance from the existing supply.

 

[Sparky Col]

Hi
It does and doesn't answer my query, maybe from my bad explanation.
I understand and thought about making the new block an TT system, but as the demand is increased I'd also like to upgrade the supply MCB slightly higher to 50amps. Upgrading the MCB would mean that the Zs is too high.
To get round this would I have to put an 100mA trip at the origin? and every circuit off the sub db an TT system?
Thanks

 

[Julie.]

Put an mcb (50A) at the supply to the toilet block along with a 100/300mA type s rcd. As "rcd provided for fault protection in accordance with 411.4.204."

Each circuit below this can be fitted with an rcbo, or rcd at 30mA for the usual final circuit protection. Coordination will be achieved as the final circuits have a lower operating current and instantaneous operation (well standard trip times anyhow) whilst the upstream rcd has a ~200ms delay .

The rcd aspect provides the ads within the required times.

No need to TT at all.

 

[Rockingit]

In addition to thoughts about RCD discrimination (follow Julie's advice above...) you might want to look closely at the cable maths for max voltage drop if you want to increase the future loading - 150m is a fair old run.

 

[westward10]

Based on a rule of thumb 2% for a distribution circuit the existing set-up is way out.

 

[pc1966]

The question of the VD on the distribution circuit also should take in to account the sort of final circuit VD values in the end block, as the goal should be under 5% for the whole lot. If it is feeding a relatively small block where you can size the final circuits for 1% drop, etc, then allowing 4%, etc, for the distribution is not unreasonable.

The extra cost for copper for the final circuits is probably trivial compared to the saving on the distribution circuit copper.

Yes, they still say 3% for dedicated light circuits but for LED lights you get much less brightness variation than ye olde filament style lights so less than 5% overall seems reasonable to me.

As @Julie. suggests feed it with a selective/delay RCD and then have (ideally) DP RCBOs for the final circuits. While you don't absolutely need DP switching, without that you don't get RCD selectivity on N-E faults, and given that Wylex and Fusebox compact RCBOs offer this at little cost it seems not too difficult to achieve.

Also if cost sensitive you can use this to allow 2C SWA and armour as CPC alone, or if TN-C-S bonding requirements apply then you might find it cheaper to run a parallel 10mm (or as applicable) CPC than to go 3C. But 3C is certainly a convenient method of achieving it.

 

[pc1966]

The other aspect to note about @Julie. suggestion is you are no longer depending on the supply MCB for fault protection, that is done by the delay RCD to meet < 5s distribution disconnection time, you can size the supply RCD for overload protection and so increase both the rating (e.g. to 50A) and make use of a higher instant trip point (e.g. D-curve) to increase selectivity with downstream MCB/RCBO.

You will never get great selectivity with MCB -> MCB as the upstream one has no short-term delay (as many MCCB do) but nevertheless you can make it better by that route.

You VD calculation should be based on the max demand, not on the supply OCPD (40A-B, 50A-D, etc). Though if you don't have any better information (not your design, quick check, etc) then using that is a reasonable starting point.

 

[pc1966]

Just re-read the OP's post and it is an existing 5C SWA so presumably a 3P supply. For 40A and 150m you are seeing 6.6V drop or 1.65% of 400V 3P so it already meets a traditional rule-of-thumb 2%

As it is 5C then the R1+R2 is basically the same (if you ignore the armour) so 0.165 ohm and with the supply Zs that is 1.03 ohms. Under a L-E fault you would get 212A (or more) flowing and while that won't meet ADS times on the MCB it is easy for the delay RCD. However, it will still disconnect on a 50A D eventually, just taking about 20 seconds.