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Topic Title: Marina Power Installation
Topic Summary: Requirement to protect all outlets with individual 2pole RCD's
Created On: 05 February 2013 12:50 PM
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 05 February 2013 12:50 PM
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oshta

Posts: 25
Joined: 30 January 2013

I have been reading up on installing power points for a marina, including the two documents below, I'm fairly new to the site but having joined would be interested in peoples thoughts on what is said and its practical application for the below scenario.

###

IET Artical
TLC Tech Page

Points of note:
-Must be TT earth if supply is PME.
-Must be able to be isolated on all poles (inc netural).
-Must be RCD protected, which shall disconnect all poles.
-One socket per berth, and no additional joints between this and he boat.
-Maximum of 4 sockets per isolation device.
-Must be 1m or more from waterlevel unless (IPX4).
-House boats have the same requirement as any other (aka, ignore)

From the TLC page:
-Cables installed in the ground but below waterline are to be considered immersed.
-PVC cables are not usually suitable for continuous immersion in water.
-Boards must be fitted with locks.
-If single phase, all of any one pontoon must be on the same phase.
-Up to six sockets may be provided in a single enclosure

Snippets of the IET aritcle:

"Regulation 709.531.2 requires that socket-outlets shall be protected individually by an RCD having the characteristics specified in Regulation 415.1.1. Devices selected shall disconnect all poles, including the neutral"

"Regulation 709.533 has requirements for protection against overcurrent. Each socket-outlet shall be protected by an individual overcurrent protective device, in accordance with the requirements of Chapter 43."

"Regulation 709.537.2.1.1 requires at least one means of isolation shall be installed in each distribution cabinet. This switching device shall disconnect all live conductors including the neutral conductor. One isolating switching device for a maximum of four socket outlets shall be installed."

####

The installation I'm anticipating is on a canal, so no salt water, no change in water level, very limited flood risk and with the expectation that the majority of the boats will be steel narrowboats. The site has a 60amp three phase supply with distribution board located in the yards workshop at one end of the plot, 40metres (mainly the length of the building) from the near side of the basin, 60m from the far side, and about 100 from the furthest boat. Total of around 20 boats, 3-4 end to end on the bank against the unit, 12 in the basin moored end on with no jetties, another 3-4 end to end against the bank beyond the basin. Width of boat is 7ft, length 50-70ft. Anyone one boat may seek to draw up to or in excess of its maximum 16amp incoming but the majority will have far less load, maybe only a fridge and some lights, or not even that.

My initial gut feeling having skimmed the regs before hand was a 32amp feed from each phase using c-type MCBs, radially daisy chaining a pair of posts each, 6posts in total, with each post containing a main switch and four 16amp b-type RCBO's feeding an outlet each. Giving 24 outlets, each capable of achieving 16amps, with a diversity of four, but also a system in which its highly unlikely that any one boat will knock out another or the whole lot, with the rcbo's being readily accessible should a trip occur out of hours. Possibly take two of the outlets for posts for the bank moorings onto the own 'sub-post' to reduce the length of flying leads and get the outlets nearer to the boats/berths. All cable outside of the building would be buried XPLA SWA, posts would be steel fabrications supplied by the boat yard, as the plastic posts some use are seen to be fragile and expensive, perhaps with reason.

However re-reading them now as a single way RCBO doesn't switch neutral that appears to be out, at which point I'm struggling get the same robustness to the supply for a sensible cost. You could replace the main switch with a 100mA RCD but it depends, but it appears very hard to individually protect each outlet with its own dual pole RCD, if that is actually what is being asked for. I have never seen this done on a marina.

Also, given there are no jetties or floating pontoons, and all the socket outlets are installed on fixed ground I presume the requirement stated in the TLC page does not apply?

A long post, but most is information/extracts for reference, so feel free to read that second!


Thanks.
 05 February 2013 01:15 PM
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AJJewsbury

Posts: 11453
Joined: 13 August 2003

I guess section 709 of the current BS 7671 should be your main reference. (I've a feeling the TLC article might be a bit out of date)

All cable outside of the building would be buried XPLA SWA, posts would be steel fabrications supplied by the boat yard, as the plastic posts some use are seen to be fragile and expensive, perhaps with reason.

What's your supply earthing arrangement? If it's PME, where are you splitting it to TT? If at the posts, which system with the post be earthed to and will it be within reach of metalwork bonded or earthed to the other system?

However re-reading them now as a single way RCBO doesn't switch neutral that appears to be out, at which point I'm struggling get the same robustness to the supply for a sensible cost. You could replace the main switch with a 100mA RCD but it depends, but it appears very hard to individually protect each outlet with its own dual pole RCD

100mA main switch is out as it doesn't provide individual protection to each socket (which 709.531.2 requires). You can get DP switching RCBOs (in both one and two module formats - e.g. http://www.meteorelectrical.co...cbo-s-mcb-s-rcd-s.html ) or you can just put a conventional RCCB in a suitable enclosure (maybe the same one as the MCBs).

My initial gut feeling having skimmed the regs before hand was a 32amp feed from each phase using c-type MCBs, radially daisy chaining a pair of posts each, 6posts in total, with each post containing a main switch and four 16amp b-type RCBO's feeding an outlet each

There's probably no discrimination between the 32A and 16A MCBs/RCBOs for high current faults (L-N or L-PE on the TN-section). HBC fuses (possibly higher rating) for the submains might be worth considering.

- Andy.
 05 February 2013 01:26 PM
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OMS

Posts: 19664
Joined: 23 March 2004

Might be worth having a look here for a it of inspiration (or is it perspiration)

regards

OMS

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Failure is always an option
 05 February 2013 01:30 PM
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Richard64

Posts: 231
Joined: 15 October 2009

Yay.
The man's back \O/
 05 February 2013 02:05 PM
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ArthurHall

Posts: 735
Joined: 25 July 2008

TT the supply before it gets near the water, as well as safety implications you may have to consider impressed current corrosion of metal parts on the boats. A local TT electrode helps to prevent this.
Some marinas want metered supplies, others want 16A sockets fused at 6A to prevent heaters being plugged in.
Ask the marina if they want a price to install water taps at the same time, the routes will be the same as the cables and plastic pipe is easy to work with.
If a boat plugs in and the socket trips there will be an argument as to where the fault is as the boat was ok last night. Sell the marina a 16A check plug so they can prove the socket is ok to the boat owner.
 05 February 2013 04:29 PM
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oshta

Posts: 25
Joined: 30 January 2013

I also have a feeling the TLC page is prehaps not bang up to data, whereas the IET article is only a year old just over. I also see no other mention of the requirment to have the boards locked, which obviously has its drawbacks from the operational point of view.

The singleway double pole RCBOs are a new on me. I guess you could always use the two-way units, as the MCG seems to, which is what you would have to do otherwise to meet the requirment, just appears a bit messy to be looping the live feeds into the bottom and would become a increasingly large post!

No tests have been done on the current earthing arrangment but the presumtion would be to TT it locally at each post. The site itself is fed from another site on the same industrial unit. One possability may be to use the steel piling of the basin side itself as the earth plate.

There is also the question of what happens if work is being done on a boat, where a supply is being taken from the workshop, ie, welding work. So I may suggest they have a TT'ed three phase supply for the basin as well and not take leads from the unit outside.

I like the idea of supplying a 'check plug' as clearly, the biggest unknown is what is in the the boat. In and ideal world they are all a-ok, safe and up to date, with an isolating transformer and there own TT system. In practice, this is likely rair, some are a right mess, and the issue of galvanic corrosion is one that is talked about a lot.

Point taken about running other services at the same time, in an ideal world I would see a duct for mains, a duct for phone/data and a duct for water, get it all pukka, but I think it would depend on the budget and percived gains as often 'less is more' in these jobs. Currently it all works, but is sketchy at best, 20years old, with non-armoured T+E and flying leads all over the place. It is RCD protected but with a single unit upstream of a submain CU in a locked outbuilding, which causes major issues in the case of out of hours trips. Have not investigated the earth to the submain of the outbuilding.

Obviously the design of the posts would have to be considerate of the implications of having water and electricity together given standard kit is only ip44.
 05 February 2013 04:49 PM
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AJJewsbury

Posts: 11453
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just appears a bit messy to be looping the live feeds into the bottom

There are plenty of DP busbars about, for both 1-module and 2-module DP devices e.g. http://docs-europe.electrocomp...9/0900766b810a95bd.pdf

- Andy.
 06 February 2013 08:48 AM
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oshta

Posts: 25
Joined: 30 January 2013

Originally posted by: AJJewsbury
There are plenty of DP busbars about, for both 1-module and 2-module DP devices e.g. http://docs-europe.electrocomp...9/0900766b810a95bd.pdf .
Yes, but your still going to end up with a 10/12way board!
 06 February 2013 08:50 AM
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oshta

Posts: 25
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Whats people thoughts about the interaction of the TT'd boat feeds and leads run out from the shop.
 06 February 2013 09:27 AM
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AJJewsbury

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Yes, but your still going to end up with a 10/12way board!

Seems about par for the course for a 4-socket supply - see OMS's link and unit DU4R4-16. Using single-module RCBOs would cut that down a lot if space is a problem for any reason.
- Andy.
 06 February 2013 10:51 AM
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OMS

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There is also the question of what happens if work is being done on a boat, where a supply is being taken from the workshop, ie, welding work. So I may suggest they have a TT'ed three phase supply for the basin as well and not take leads from the unit outside.


I think you need to decide if the workshop supply taken outside the workshop building falls under the requirements for marinas, construction sites (under BS 7375), mobile or transportable units, restricted conductive locations, exhibitions shows and stands or something similar.

In every case there is a good argument for the designer to both avoid the use of a PME earthing terminal and to provide all pole RCD protection not exceeding 30mA, additional supplementary bonding where appropriate, along with good workmanship and material selection. There is also a good argument to look at why we have Reg 709.553.1.11

In general, it's going to present a tough location and you really can't be sure of maintaining any kind of equipotential zone so I would avoid PME and I would put in RCD protection (ideally more than 1 to avoid potential failure) - so something like 300mA TD (adjustable shunt trip) supplying 100mA TD and 30mA non TD devices. The idea being that even if your local additional protection type device fails, the upstream devices wil still give you ADS in a "reasonable" time - subject to what Ra you can achieve.

You could also of course bang in an isolating TX for each of the industrial sockets you want as an alternative.

If you are designing this lot, most of it is going to be "rule based" directly from BS 7671 - you can use the fundamentals of protection for all of the particular issues you have that are not strictly "Marina", to come up with a well engineered solution at realistic cost.

Regards

OMS

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Failure is always an option
 07 February 2013 02:22 PM
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oshta

Posts: 25
Joined: 30 January 2013

Originally posted by: OMS
I think you need to decide if the workshop supply taken outside the workshop building falls under the requirements for marinas, construction sites (under BS 7375), mobile or transportable units, restricted conductive locations, exhibitions shows and stands or something similar.

Well maybe. However which title it falls under doesnt not effect the risks of various options.

Welding work on a boat in the basin would be a restrictive mobile construction site in a marina, so you could say anyone one, or almost all of the above could be relatated to it.

At which point in my eyes the question ort to be 'Whats the chance of personal injury (shocks or death) or damage to equipment (welder, boat electronics, etc) and what is in place to control that' both from the point of minimising risk, and controlling legal or professional obligations.

And then, rightly or wrongly, we have bottom line of cost in that the worse case in almost every way is that the whole job is thrown out on cost and no improvments are made.

As you say, a lot of it will be rule based, such as requiring DP RCDs on each outlet but within that there are diffrent routes open, such as using single or twin way RCDs. Previously there could be one RCD per group of outlets, but not under the 17th ed.
 07 February 2013 02:32 PM
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AJJewsbury

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Well maybe. However which title it falls under doesnt not effect the risks of various options.

But it helps you to group the risks and work out (from a BS7671 perspective) what needs to be done to address them.

Welding work on a boat in the basin would be a restrictive mobile construction site in a marina, so you could say anyone one, or almost all of the above could be relatated to it.

I'd go for the all-that-apply option. So you'd be looking for a solution that simultaneously met several parts of part 7.

- Andy.
 07 February 2013 02:48 PM
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OMS

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Originally posted by: oshta

Originally posted by: OMS

I think you need to decide if the workshop supply taken outside the workshop building falls under the requirements for marinas, construction sites (under BS 7375), mobile or transportable units, restricted conductive locations, exhibitions shows and stands or something similar.


Well maybe. However which title it falls under doesnt not effect the risks of various options.

I think you misunderstand me - addressing risks in everyone of the above would direct you to a solution that's been as derisked as you can achieve within the context of BS 7671.


Welding work on a boat in the basin would be a restrictive mobile construction site in a marina, so you could say anyone one, or almost all of the above could be relatated to it.

OK - so decide on a solution to each (or most) of those and then see what the common theme is - if you don't have avoidance of the PME earth and protection by 30mA RCD's in that list then you've missed my point totally.


At which point in my eyes the question ort to be 'Whats the chance of personal injury (shocks or death) or damage to equipment (welder, boat electronics, etc) and what is in place to control that' both from the point of minimising risk, and controlling legal or professional obligations.

Well the liklihood of hazard is high (environment, frequency etc) and the hazards are easily identifiable (shock, fire burns) - so what controls do you think should be in place - see above ?


And then, rightly or wrongly, we have bottom line of cost in that the worse case in almost every way is that the whole job is thrown out on cost and no improvments are made.

Try a google for the absolute duty in EAWR and then from there also consider reasonably practicable.

A definition from the case of Edwards v National Coal Board:

"'Reasonably practicable' is a narrower term than 'physically possible' ... a computation must be made by the owner in which the quantum of risk is placed on one scale and the sacrifice involved in the measures necessary for averting the risk (whether in money, time or trouble) is placed in the other, and that, if it be shown that there is a gross disproportion between them - the risk being insignificant in relation to the sacrifice - the defendants discharge the onus on them."


As you say, a lot of it will be rule based, such as requiring DP RCDs on each outlet but within that there are diffrent routes open, such as using single or twin way RCDs. Previously there could be one RCD per group of outlets, but not under the 17th ed.

That's a matter for the designer, based on addressing risk and compliance with BS 7671.



Regards

OMS

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Failure is always an option
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