Surge protection devices and up to date requirements.

[John J]

I've been asked to run an eye over a proposed shipping container job at a local pond. The electrical supply will be via an underground 16mm 3 core PVC/SWA from a new service cabinet supplied via underground from the street.. The service has already been laid on to the cabinet with a pme terminal provided but no meter etc as yet. I'm thinking a BS88 switchfuse to protect the sub main. At the container end use an insulated gland and a 100mA delayed action RCD with an earth rod to form a TT installation. Individual circuits to be all RCBO 30mA fast acting. Anticipated to supply the following - light 6A, frost protection tubular heater 10A, industrial socket 16A, couple of 13A sockets for small tools 20A, FCU for CCTV system 16A and a 32 type C provision for a fountain pump for aeration of the water in hot weather at a later date. Under latest versions of BS7671 would there be a requirement for an SPD and if so would it be best located at the BS88 switchfuse or in the consumer unit in the container? The pond guy will also be looking for a contractor near Brough, East Yorkshire to give estimate for the job if anyone wants to bid.

[Theo]

What kind of location is the site in? Urban, rural? Poled cable runs nearby, or all buried? There are some complicated formulae in the regs for calculating the likelihood of surges, but at the end of the day it depends how much you're likely to have lightning activity get into the local grid, and what you have that might be damaged by it. It's a risk/cost tradeoff. I don't think SPDs are mandatory, merely recommended at this stage.

From your list I'd guess the CCTV might be a concern, but the others are relatively basic loads that might not be fried by the kind of voltage surge an SPD can absorb (if you get a direct lightning strike nothing will protect you). Although it depends how much electronics there is: a pump could just be some coils, or it could be a fancy thing with control electronics, which might not handle surges.

IANA electrician but the idea is the SPD should be as far upstream as possible - you want to absorb the surge before it affects anything downstream. You can do it in the consumer unit, but you need to keep the wiring short (typically SPD CUs have it wired directly next to the main switch).

Not sure if that's Adam's patch...

Theo

[John J]

It's a big village situation so urban. The 11kV cabling supplying the area outside the village is overhead on wooden poles. As far as I can ascertain the street cables from the 11kV to 400/230 are all underground and the 11kV cabling around the area goes undergound a mile or so distant from the substation. From an admittedly 60 year ago hazy memory of a transmission & distribution course, travelling surges on overhead lines are reflected back up the overhead line from the change of impedance between overhead conductors and cables so my thoughts would be very low level of surge risk. I haven't heard of a single real surge situation in the area apart from at one farm property outside the village twenty-five years ago when a lightning strike to a tree in a garden behind the house took out all the electronics in the house (TV, boiler controls, cooker and intruder alarm.). From what you've said I'm inclined to think any SPD (if required) would be best located at the meter cabinet end in at the termination of the sub-main into the BS88 switchfuse. The CCTV system can rely on its own internal surge protection and if there is a local strike causing damage to it via the camera wiring it can be replaced at reasonable cost. The pump will be a straightforward contactor operated induction motor with a timer to run during the day to pump water up into a fan cascade to introduce air to the pond water to boost dissolved oxygen when/if temperatures rise and fish show signs of distress (as happened during a couple of summers in the last decade. During these incidents a portable diesel pump was employed together with a firehose nozzle pointing upwards to create a cascade and most of the big carp and other fish lived.)

[ARW]

I have a few suggestions/comments.

The cost of a risk assessment to show that you do not require a SPD costs more than a CU with a built in type 2 SPD.

You do not need a 100mA RCD time delayed switch if all the circuits on the CU have 30mA RCD protection. And 100mA S type RCDs are not cheap.

The CCTV might need to be in it's own enclosure with a separate (50W) heater and stat.

Cheers

[Animal]

no need for the 100mA S RCD when the circuits are RCBOed

the last 2 are far too high. 6A is probably more than enough for both.

[John J]

The pond covers about an acre. CCTV will be somewhat extensive with multiple pole mounted cameras that will each have frost protection and long range infra red illuminators hence the generous power provision. The pump for cascade/aeration when used will be pumping a lot of water to a significant height. A 5hp 2" portable diesel pump was employed in recent hot spells and threw a jet of water up about twenty feet in the air and nearly 50 feet out over the pond to fall back into the water carrying air into the pond. I'm not involved with the pump as a fountain contractor will be engaged at some point. I'm making an estimate of what might be appropriate. The value may change accordingly. (Or the estimates might result in a rethink of course)

[ARW]

The type 2 SPD would be better located at the DB not at the meter.

The CCTV and PSUs etc can be fed from a type 3 extension lead or similar if additional protection is required.

Another thought is why not use 2 core SWA instead of 3 core SWA?

[John J]

That would have been logical but I was asked to look at the pond electrics idea after the pond people had bought the cable based on "advice" from a trade counter, dug the trench and installed it last summer then asked me for my input. Don't know what current pricing difference it would have made but it's water under the bridge now. I think it's fortunate they've not ended up with something undersized. If I'd been involved at the beginning I'd have pressed them to have the service and meter located at/within a couple of metres of the container then avoided having sub main altogether but hey ho, it is what it is. What's the reasoning about having the SPD at the DB rather than at the source Adam? I'm curious why they have been introduced as a "necessary" extra when over voltage surges have not been a noticeable problem in urban areas as far as I'm aware. Is it another part P exercise?

[ARW]

page 9 as to location of SPD.

The introduction is due to the IET 18th edition regs requiring them in certain places.

A domestic property usually has a get out clause that does not require them even if you did not bother with a risk assessment.

Your installation would require a risk assessment.

All the best with it. And if I go back to HMP Humber or Swanland Primary school in the New Year I'll call in for a look if you want and make any other suggestions based on what similar jobs I have done.

[Robin]

Not like Part P. With that and other legislation (e.g. for smoke and CO2 alarms) we at least get an impact assessment and can see what costs and benefits they claim. With BS 7671 you get whatever they choose to make public and I've seen no impact assessments/cost-benefit analyses of SPDs or anything else. So on the one hand we have manufacturers, retailers, electricians and insurers who might all be keen to see SPDs mandated. On the other hand we have end users who foot the bill for SPDs (or new CUs with SPDs). And the balance is judged by an unaccountable body outwith the FoI Act (having seen off the attempt in 2011 to bring them within it).

[John J]

Thanks Adam. Unfortunately I don't have access to the container but if things develop I'll let you know. ??