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Topic Title: RCD usage in floating fish farms
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Created On: 12 July 2013 09:56 AM
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 12 July 2013 09:56 AM
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haveyoubooked

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Hi there, i'm an electrician in the Northern Highlands and have got to an issue at a fish farm I could do with some help on.

I've built four panel boards with 10x 16A single phase socket outlets in each, protected by MCB's. These 40x 16A socket outlets supply the underwater 400W lights.

Each board is fed via a 4mm 5 core HO7RN-F cable to a distribution panel. In here there is a 4 pole Main Isolator feeding through to 4x C32 3 phase breakers coupled to 4x 4 pole 30mA RCD's. The idea is that then we only lose 10 sockets in the event of a fault.

However this distribution panel is fed via a 16mm 5 core HO7RN-F cable to the generator. On the fish farm everything is exposed to the elements, so all the panels are IP65 plastic enclosures inside steel enclosures. This 16mm feed cable is 12m long, protected by ducting and links on to the generator, which is on a raft, tethered to the floating fish farm....

The question:

Is it OK to put a time delayed 100mA RCD in the generator for this feed cable, so as to discriminate in the event of a fault. It previously ran a 30mA in the generator, so if the RCD trips, all lights go off and a few hundred thousand fish start to suffocate. With my proposed method, all the sockets will still be protected by RCD, there will just be 1 15m length of well protected cable hard wired at both ends that doesn't have 30mA protection, but much improved functionality

Sorry it is wordy, trying to explain the circumstances for helpful comments.
 12 July 2013 11:31 AM
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haveyoubooked

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Quick update, finally got through to NAPIT technical and they said there is no requirement for RCD protection, only what the generator manufacturer and the lighting manufacturer stipulate. I'm still uncertain as to the route to go here, would appreciate further comments.
 12 July 2013 11:59 AM
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OMS

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Don't you have a totally floating system (both electrically and physically) - you have no N-E reference and no probablity of any RCD working.

Regards

OMS

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Failure is always an option
 12 July 2013 01:13 PM
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AJJewsbury

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Sounds like a reasonably hefty generator - wouldn't something of that size normally have a N-PE link? RCD should then work for faults L to c.p.c or exposed-conductive-parts regardless if the entire system is referenced to true earth or not (like some of the section 717 arrangements). 'Additional protection' might be limited by the lack of true earth reference, but then any shock is likely to be to pontoon/water rather than true earth anyway.

- Andy.
 12 July 2013 01:41 PM
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OMS

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That assumption really needs to be checked Andy - in my experience, larger sets almost certainly don't have the link fitted - it has to be provided either at the gen set or at the switchboard.

If it's present, then yes, RCD protection can be enabled and the usual rules on discrimination by rating and time delay can be deployed.

Regards

OMS

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 12 July 2013 02:12 PM
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AJJewsbury

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From a regs point of view it's an interesting setup. I presume fish count as livestock, so 705 would seem to apply; the floating/pontoon system would suggest that parts of 709 should be taken into account and the "on-board" generator and lack of true earth makes the setup more like a section 717 arrangement. I've not noticed anything in any of those sections that would object to a 100mA S type RCD on a distribution circuit.

I do wonder what the cumulative leakage current might be like with 40-odd submerged fittings in rough environment might be though. A couple of mA each and the 100mA might go while all the 30mA units held.

Also if you've got 4 banks of 10 sockets, it sounds a bit like you're presenting the 3-phase generator with an unbalanced load - although I might have mis-interpreted that.

- Andy.
 12 July 2013 02:27 PM
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AJJewsbury

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Why do you always think of something else after pressing 'reply'??

Not special location at all, but 411.3.3 calls for 30mA RCD protection for 'mobile equipment <=32A outdoors' - would in installation on a floating platform that presumable can move with the tide and/or be towed about count as "mobile" ? I'd have hoped not, but it might be worth asking the question.

- Andy.
 12 July 2013 02:47 PM
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OMS

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Yep, I think I'd be selecting the relevant points of all the special locations - but basically tiered RCD protection is the order of the day.

The generator should have a stated curve for imbalance - assuming it has reasonable balance up to first load step (say about 50% of full output, it should be able to tolerate about 30 - 40% imbalance between phases - but again, worth a check.

300mA adjustable TD up front, 100mA TD at the next tier and more subdivision, with 30mA final circuit RCBO's might be the order of the day.

Regards

OMS

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 12 July 2013 04:58 PM
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haveyoubooked

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Hi lads, thanks for the flood of responses.

Well with regards the load balance... In each "satelite" box there are 4 sockets on one phase and 3 on each of the other 2 phases. The extra socket is on L1 in the first panel, L2 in the second panel, L3 in the third panel and so on, so we are only ever around 2A out of balance maximum on the whole installation. Its not an ideal setup, but the physical location of the cages dictates it has to be that way.

Regarding the earthing: Fish farms are fairly new to me, i'm assuming its getting its earth from the Loch body of water passing through the steel frame of the pontoon, or through the hefty anchors, so in response to OMS, although not intentionally earthed, it isn't earth free either so has to be treated as earthed. Interestingly on that subject, one of the lads did say that one of the lights safety ropes came off leaving it to break away from the cable and its exposed live end submerged in the Loch...nothing tripped - possibly down to very pure water?! However in the past 6 months i've been modernising the group of fish farms, i've seen plenty of RCD's trip, and on the test they are always well within the time boundaries.

Is there any recommendations you would give regarding earthing a floating pontoon? Its anchored by about 15 anchors. With regards to the generator, the farmee hires in a 30Kva generator each year for the 4 months that the lights are required. As such there is no way of me telling what its earthing arrangement is. It is towed out to the pontoon on a steel barge and tethered to the pontoon. (It might be worth mentioning there is a "small" 63A feed that comes from the shore on a 500m 16m 5 core cable that sits in the bottom of the Loch. This currently feeds the feeding system, the amenities for staff and communications equipment which is required year round. I didn't install it, but having looked at the feed origin (a rotten shed on the side of a mountain with meter, cutout, and a 4 pole 63A 30mA RCBO) it appears to have a connection to the incoming earth. It doesn't have a pme sticker so probably is TN-S. However it was done by a reputable company 6-7 years ago, so i'm at a loss as to whether its safe/legal/functional to leave that earth connected??))

I've just spent a couple of days building panels ready to install... So far the tiered protection is:
100mA TD RCD purchased to fit in the hired generator feeding to distribution panel
30mA RCD x4 fitted in distribution panel to feed satelite panels. The 30mA RCD fitted at this point means the rubber cables that feed the power to the satelite boxes are also protected if one is damaged (unlikely as i'm putting ducts in but still possible). Then only 10 sockets go if there is a problem. I wanted to put a 30mA RCBO in for each socket, but the cost of 40+ RCBO's, plus panels to put them in plus the fact the rubber feed cables would be left with less protection proved prohibitive.

So questions remaining...
What to be done with on shore TN-S connection?
What to be done (if anything) about the earthing of the pontoon (around 200m long by 20m wide)
Is it OK to pop my 100mA TD RCD in whatever generator arrives?

Thanks so much for all the help. It's a few years since I was sat in college doing the theory and its a field (fish farms) there doesn't seem a massive amount definite advice on, so answers are very much appreciated.

Regards
Rory
 12 July 2013 05:12 PM
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OMS

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OK - so it's essentially the same as a boat moored in a marina - so section 709 is the starting point.

That allows you bring an earthed neutral and protective earth via shore RCD to the pontoon RCD protected circuits - so you need to look at the TD side of the shore supply.

For the generator installation, essentially the ship side power but with the added earthing of the anchors, it's likley that the anchors are an effective electrode, so the gen set needs a N-E bond and a source RCD - from there it's basically a TT system with the anchors (and the shore side CPC) acting as the electrode.

Your design now sounds pretty reasonable - just seek a confirmation at each hire that the set is in fact N-E bonded and not floating - I'd put a label at the connection point to that effect.


Regards

OMS

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 13 July 2013 11:50 PM
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SMLaurenson

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Are these steel cages or Polarcirkels? We have done a lot of underwater lighting for fish farming, in saltwater rather than fresh. Our standard installation is a distribution board with C25 mcbs feeding 4pole 100mA s type rcds, one for each cable running to the cages from the feed barge (usually a steel or concrete construction) which contains the generator, feed system, welfare facilities. If it is plastic cages, a typical configuration might be two rows of four circular cages on a 50m grid in which case we would run two 5 core 6mm steel wire armours out in feed pipes floating on the surface. There are manufacturers doing heavy polyurethane sheathed cables similar to that used for the lights themselves but the costs are high. At each cage we install a polycarbonate enclosure with 2x 16A 2pole rcbos and 2x IP67 16A sockets for the lights. The swas are looped in and out at each cage with the sockets being spread over the phases. The swas are terminated using IP68 swa glands and the pipes are fixed to the cages and the cables attached with clamps,leaving a slack loop to allow for movement. Individual rcbos for the final circuits are preferable as the fish get spooked by the lights going on and off. A fish that's lost its appetite is costing more than the price of an rcbo! Every ton of feed they don't eat is a ton of weight they don't gain. We have done configurations of lights from 400w to 1000w metal halide in varying quantities. The largest underwater installation was 80kw of lighting over 10 cages for cod. This was on steel cages which are a doddle compared to "suicide circles".
 14 July 2013 12:19 AM
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sparkingchip

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Two general interest questions.

Am I right in assuming the lights are used to extend daylight to encourage feeding the same as chickens in sheds?

Do the electricians get their own boat and is it a good one?

Andy
 14 July 2013 09:32 AM
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SMLaurenson

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Hi Andy,

The lights are switched on 24 hours a day from around December until May to prevent grilsing (early maturing). If they mature early they don't gain weight so readily and the end product is lower quality. As for the boats, you would generally be taken around by the farm staff in either a twin hulled steel workboat if you are lucky or an open fast boat if you are not!
 14 July 2013 09:03 PM
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weirdbeard

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Originally posted by: haveyoubooked




. With regards to the generator, the farmee hires in a 30Kva generator each year for the 4 months that the lights are required. As such there is no way of me telling what its earthing arrangement is. It is towed out to the pontoon on a steel barge and tethered to the pontoon. (It might be worth mentioning there is a "small" 63A feed that comes from the shore on a 500m 16m 5 core cable that sits in the bottom of the Loch. This currently feeds the feeding system, the amenities for staff and communications equipment which is required year round.


Hi, just a thought but do you have a rough idea what the actual consumption is for the existing shore power? Just wondering if it would be possible/ worth considering upgrading the 500M cable to the shore so the whole lot was on the mains, with a shore-side backup genny?

I have a figure for a 30kva genny at 3/4 load that says fuel consuption is expected to be 5.2L/h, so at 60p a litre of red diesel, thats £8386 ish in diesel cost for the 4 months - a good sized 500M of cable wouldn't cost much more and has potential benefits such as decreased running costs of the lights, increased reliabilty, and reducing the environmental risk of carting thousands of litres of fuel across the water
 15 July 2013 08:04 AM
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haveyoubooked

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Thanks again on the reply's, in order of questions:

They are the steel cages, not polarcirkles, so easier there. The reason that we haven't gone with Polycarb boxes and exposed weatherproof socket outlets is that there has been problems in the past with seals breaking down and continuous rough practice of the lads on the farm putting standard plugs on rather the the locking collar IP rated plugs on. With the steel boxes/internal IP rated enclosures, the chances of water ingress is fractional, even after 10 years once the seals start to deteriorate. That said, electrically, your method is superior, and I would like to steal some of that method for the next one we do in a couple of months.

We use a farm boat (in this case open topped, 75hp = quick crossing, and need for a pack of bin liners and shares in WD40 for the tools at the other end!)

Finally with regards the cable, we are looking at using between 80-85A load from the underwater lighting once the farm is up to full speed + there's already a 30A+ load from the feed system/compressor/hot water dispenser (for vital tea!) heaters, computer systems and pontoon/enclosure lighting). So we would be looking at a total load of about 110A. If we were clever with the loading/start up sequence, we could probably get it down to 100A with diversity etc. However its the Remote Highlands so there's power cuts a plenty, and we are right on the limits of the supply size. And x1 25mm 5 core H07RN-F cable (theres currently a 16mm version in the Loch at the moment) would give a CCC of 114A with a volt drop of 1.7 mV/A/m giving a massive 85A volt drop. So it would have to be a huge 95-120mm cable required to give us a decent supply according to the regs. I wouldn't even like to think what 500m of 120mm semi-flexible cable would cost. But I can see the idea your getting at, and until i'd done a few calcs, I was thinking along similar lines.
 15 July 2013 10:21 AM
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haveyoubooked

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85A voltage drop hmmm might have meant 85V drop there!
 15 July 2013 10:51 PM
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sparkingchip

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I was still thinking about the types of cages when you mentioned semi-aquatic marine mammals, " there has been problems in the past with seals breaking down " at which point I expected you to say cages!
 16 July 2013 07:36 PM
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weirdbeard

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Originally posted by: haveyoubooked


Finally with regards the cable, we are looking at using between 80-85A load from the underwater lighting once the farm is up to full speed + there's already a 30A+ load from the feed system/compressor/hot water dispenser (for vital tea!) heaters, computer systems and pontoon/enclosure lighting).


Hi hyb, just wanted to check.....are you planning to have more lights than the 40 x 400s you mentioned?....

I made that 29 ish amps per phase..

...still wondering if the existing shore mains power is 30+ amps continuous(what's the hourly average from the electric bill? ) - or more intermittent loads that sometimes may up to 30+, as to whether you could get a fairly reasonably sized cable to replace the existing one?




Reading the original post again I'm thinking that the advantage of looking to upgrade the mains supply and having a shore side back up genny is tackling the issue of supply reliability at the source, rather than trying to design in reliability further down the line with possibly unnecessary RCD devices.
 16 July 2013 10:39 PM
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haveyoubooked

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Hi weird beard,
Yep your right there is talk of more lights. 40 "should do" but I know there are still another 8 cages that aren't lighted. So i'm just trying to plan ahead a little as the demand for the cages tends to fluctuate depending on business each year.

Also, the 30A is just whilst everything is on. However, the lighting is a load we can't diversify (is that a term?!) for as it runs 24 hours a day for a few months. BTW the lights specify 3A startup, 1.8A run, so i'm allowing 2A run per fitting as some are pretty ancient. With regards the 30A load, the heater (its North Scotland) water boiler, feeder system and computers are always running, though not admittedly some of the large motors and compressor in the feed system as these only come on at feeding times.

So it would still be a massive cost to upgrade to a large cable.Out of interest though what neutral current do you expect I would see, realistically from this installation?
 17 July 2013 05:42 PM
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weirdbeard

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Originally posted by: haveyoubooked



So it would still be a massive cost to upgrade to a large cable.


I would say 'a good investment' for the customer, and a tidy mark-up for you!


Out of interest though what neutral current do you expect I would see, realistically from this installation?


How many lights will there be?
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