With 18th Edition kicking in from January 2019 comes a tightening of the requirements for surge protection, especially on commercial premises, so let me introduce you to the SY2-D, a low cost means of fending off spikes and surges for domestic or commercial applications.

For those of you who prefer moving pictures to reading words, you’ll be pleased to hear this article is available in video form on YouTube…

For anyone still reading, let me start by saying I was at the Elex Show in Coventry last month, and this is where I came across the stand for Surge Protection Devices Ltd. who, I must admit, are a brand I am not previously familiar with.

 Surge logo

Their representative on the stand certainly knew her products and was doing an impressive job of fending off questions from several directions at once from people like me who traditionally haven’t had a lot of exposure to SPD's and don’t know much about them. I found their SY2-D SPD product in particular to be rather exciting, so I went off with her brochure intending to find out more, and indeed I wasted no time by popping into City Electrical Factors that very afternoon who shrugged their shoulders at the mere mention of SPD’s. Then I went into Edmundson who did exactly the same.


Denmans however do list a surge protective device, model SS40, and badged for use with their Curve and Steeple boards, but I’m pretty sure I’ve also seen it with Lewden branding which is no surprise as I think these all come off the same production line, although I may be wrong...

denmans ss40

...anyway, at the time of writing the Denmans website shows no stock of the SS40, so these are either selling like hot cakes, or they’re not shifting in any quantity at all. I suspect the latter to be honest.

The general lack of availability and product awareness at the sales counter tells me that these large suppliers aren’t selling a lot of SPD’s at this time. I guess most jobbing sparkies, especially those who mainly work domestically, are not routinely installing SPD’s, which is no real surprise as 17th Edition and earlier didn’t cover the topic too much. Off-the-shelf consumer units, whether empty or fully loaded, are supplied without an SPD and often with little consideration for the retrofit of such, and in the competitive market of domestic installations where dickhead chancers, cowboys, unqualified builders and such are spunking consumer units onto walls with little regard for the wiring regulations and at a price that undercuts legitimate installers, well it’s hard to justify adding in extra part costs unless the client has expressly requested such additional functionality.... which they generally don’t when it comes to SPD’s, as Joe Public out on the street probably has no idea these devices even exist. Besides, I suspect there are relatively few damaging surge events here in the UK, or at least I’ve not been called out to deal with any in my time, but 18th Edition now requires us to have these things on our radar.

Before we get into details, it’s important to understand that there are three different kinds of SPD:

Type 1 would be located at the main incoming position and protects against a direct lightning strike.
Type 2 would be at the distribution board and protects against indirect lightning strikes.
Type 3 would be fitted to a final circuit or at an item of equipment to protect locally against switching overvoltages, that is the kind of overvoltage that may be caused by large inductive loads such as motors and air conditioning.

Combination units can be sourced, and according to the manufacturing blurb supplied with my model, a type-2 SPD is okay for installations fed by underground cables, but a type 1-2-3 combination device would be required for installations fed by overhead power lines, regardless of whether that’s single or three-phase. In this article, I’ll be playing with a Type-2 device as I’m on an underground PME connection, so this is what I’ll be installing for my property. Incidentally, the SY2-D device is suitable for both TN and TT installations.

18th Edition Regulation 443 is what we’re interested in, and in its own roundabout way it says that if you’re covering an installation from surges caused by atmospheric conditions, then you’ll likely also be covering it against switching overvoltages which are normally lower in amplitude. This means that by installing an SPD into my consumer unit, I should be protecting my own installation adequately enough for whatever me, the bloody wife, or mother nature can throw at it, but as 18th Edition bites in, am I actually required to be installing these things for my clients as a matter of course? Well let’s quickly look into that before we get the screwdrivers out.

Regulation 443.4 states ‘Protection against transient overvoltages shall be provided where the consequence caused by overvoltage could:”

(i) result in serious injury to, or loss of, human life, or
(ii) result in interruption of public services and/or damage to cultural heritage, or
(iii) result in interruption of commercial or industrial activity, or
(iv) affect a large number of co-located individuals.

For all other cases, a risk assessment according to Regulation 443.5 shall be performed.

So, if you want to get out of installing SPD’s, you turn to Regulation 443.5 which gives you a formula for calculating the risk level. If the result is greater than or equal to a thousand, then you don’t need surge protection, but if it’s under a thousand then you do.

Here’s the formula:

CRL = fenv/(Lp x Ng)


Fenv is an environmental factor selected according to Table 443.1
Lp is the risk assessment length in kilometres, and
Ng is the lightning ground flash density measured in flashes per square kilometre per year relevant to the location of the power line and connected structure.

As far as mathematical formulas go, this doesn’t look too terrible; we just have three factors to find. Let’s start with fenv and refer to table 443.1 which seems simple enough in that there are only two numbers to consider.


Table 443.1 - Calculation of fenv
Environment  fenv
Rural and suburban environment  85
Urban environment  850


I’m not sure at which point you differentiate between urban and suburban installations, but for my own house I’m going for “rural and suburban” seeing as I’m on the outskirts of a market town with a golf course and green fields immediately behind me, so my value for Fenv is 85. God only knows where they get 85 and 850 from, but who am I to argue?

Let’s skip then to Ng, the lightning ground flash density. We can find that in Figure 44.2 of the Big Blue Book where a map shows us lightning flashes per square kilometre, and as you’d expect, the south east has a higher density of lightning strikes than up t’ north as indicated by the darker areas.

Figure 44.2

Ng varies from a factor of 0.1 for areas with a low volume of thunderstorm activity, up to 1.4 in some parts of Suffolk and Lincolnshire. Where I am here in Warwickshire, I can see that Ng is a factor of 0.8.

Lovely stuff. All I need now is Lp. Let’s see, according to page 103 of the Big Blue Book…. "The risk assessment length Lp is calculated as follows:"

Lp =2 LPAL + LPCL + 0.4 LPAH + 0.2 LPCH (km)


LPAL is the length (km) of low voltage overhead line
LPCL is the length (km) of low voltage underground cable
LPAH is the length (km) of high voltage overhead line ..and
LPCH is the length (km) of high voltage underground cable

What the fu….

Dammit, I knew this was all looking too easy. Okay, well let’s define what these things are.

Let’s start with LPCL as that’s the bit connected to my house, indeed it’s the length of the low voltage underground cable between my intake position and the nearest LV transformer, illustrated here as being pole mounted although that may not be the case of course.


Next, we have LPAL which is the length of the low voltage overhead line. Then we have more to factor in to LPCL where the cable runs between the high and low voltage transformers.

LPCH is the length of high voltage underground cable, and LPAH is the length of high voltage overhead line. Now, different installations will be fed in differing ways, it’s not always going to look like the pretty picture above, but to perform this calculation, I need to know these distances. Unless you’re a developer building a new estate where these factors are known because you’re involved in them physically being trenched in on your watch, then you’re not going to be plugging any numbers into this formula with any kind of accuracy. Regulation 443.5 says that where distribution network lengths are totally or partially unknown, then LPAL can be assumed to be the difference between LPCL and one kilometre, so if the underground cable to my house is 100m long, LPAL can be assumed to be 900m long.

Anyway, I don’t know how long my underground cable is. I’m not even sure which transformer I connect back to on this estate. I have measured an external impedance of 0.08 Ohm here which may indicate a length of between 35 and 55 metres depending on whether my incoming cable has a CSA of 16 or 25mm to my 60 Amp PME head, but I’m not aware of a transformer that close to my property, although there are some buildings on the golf course which might house such.

So, this formula is damn near useless for most of us norms it seems. For shits and giggles, let’s go with a figure of 55 metres for my underground cable and for the value of LPCL just to try this formula out. This means I can apparently assume LPAL is 945 metres, but assuming numbers for a formula seems to defeat the purpose of doing any maths, I may as well just make up the result as I’ve no confidence in whatever the answer will be anyway, but we’ll keep floggin' the formula and see where it goes.

As I only have numbers for LPAL and LPCL, I’m disregarding LPAH and LPCH. That’s what they seem to do in a few worked examples in the blue book, so anyway, as the formula works in kilometres and we have values in metres, we’ll shuffle the decimal points around, so take 0.945 multiply by two and add to 0.055 which gives us… 1.945 for Lp.

Okay, let’s plumb all that into our original formula for the calculated risk assessment:

CRL = 85 / (1.945 x 0.8)

85 divided by the product of 1.945 multiplied by 0.8. Will it be higher or lower than a thousand?

Well, obviously it’s lower; it's 54.6. Actually, it seems to me like it pretty much always will be lower than 1000, unless that is you’re living in deepest Scotland with the supply transformer located right next door.

If you don’t know the cabling distances to accurately use this formula to get out of installing surge protection, then to comply with 18th Edition, you’re going to be fitting SPD’s as a matter of course in commercial installations. Why commercial installations? Well, there is a silver lining for domestic applications. Going back to Regulation 443.4, a get-out-clause is given for ‘single-dwelling units’ where the ‘total value of the installation and equipment therein does not justify such protection’. So, you can sidestep surge protection when working on a single house, flat, apartment etc., nonetheless, it’s best to discuss with the homeowner the benefit of forking out for the installation of surge protection and have them make the decision.

An elderly couple with few high-end electrical appliances in an urban location and at a low risk of a lightning strike may feel it’s just not worth the expense, whereas a gadget freak in a rural environment may feel more inclined to shell out a bit extra to see their installation and appliances suitably protected, and here I’m looking at the practicalities and costs for providing that protection, at least on a single phase dwelling, so enough with the maths, let’s have a look at the SY2-D and how it’s installed.

Unlike arc fault detection which is applied to individual circuits, a type 1 or type 2 surge protector is connected in parallel with the entire installation meaning only one needs to be fitted.

type2 install

In the case of this Type-2 model, it’s the size of a standard MCB and so can sit on a DIN rail at the CU or distribution board, however the bottom terminal is for protective earth: this model does not sit on the busbar, so in retrofit applications the busbar may need to be cut down if practicable.

The protective earth terminal connects to the MET in the consumer unit, while line connects to a 16-32A breaker. That breaker can be one dedicated to the task, or if space is sparse, perhaps use one that also serves another circuit. Obviously, neutral connects to the neutral bar associated with any RCD the breaker is hanging off. It’s earth leakage free, so shouldn’t trouble any RCD’s located upstream, at least not in the case of this particular Type-2 model, although Type-1 SPDs are a different kettle of fish according to the blue On Site Guide. Regulation 534.4.7 requires any upstream RCD to have immunity to surge currents of at least 3kA, but devices compliant with BSEN61008 and 61009 do satisfy this requirement. Effectively, this SPD is treated as a final circuit in its own right. If there’s no space in the existing CU to accommodate it, then it can be installed in another enclosure adjacent to the CU. Regulation 534.4.10 requires a type 2 SPD to have an earthing conductor with a CSA of 6mm2 and for its supply conductors to be appropriately rated for the breaker in use, but to be of a size of no less than 2.5mm2, so you can use 2.5mm2 for a 16 or 20A MCB, maybe bob up to 4mm2 or 6mm2 if hanging off a 32A, although I probably wouldn't bother. For a type 1 SPD, minimum CSA is 16mm2 for the earth and 6mm2 for the supply conductors.

When it comes to locating this device into a CU, we have to bear in mind new Regulation 536.4.203 which disallows components to be installed in consumer units unless type tested by the manufacturer, so retrofitting one of these into an existing enclosure could fall foul of that. I’m told that the SY2-D has been type-tested for Lewden boards and for CEFCO, the manufacturing arm of CEF, so presumably that means the likes of MCG, M2 and Proteus, but I’m not terribly sure. If in doubt, check with the manufacturer or install in a separate approved enclosure, although personally I’d probably accept the risk and maybe note it as a deviation on the certificate so long as it fits on the DIN rail and the enclosure seals back together with no deformity or modification required. When it comes to mixing components in a CU, it’s often busbar misalignment you need to be wary of, but that’s not a factor here as this device doesn’t sit on the busbar. Anyway, that’s up to you to decide, but if mounting in an enclosure externally to the consumer unit, then keep it close, Regulation 534.4.8 prefers you not to exceed 50cm on the supply wiring, and definitely not to exceed one metre.

Personally, I’m happy to fit one into my own home's Wylex board, type tested or not. How do I know it works? Well, I don’t, there’s no way for me to reliably test it. In fact, I’ll have to periodically visually check it to ensure the indicator windows still show it as being functional. Some models come with an audible alarm or with contacts to connect to such, but not this one. Incidentally, you also have to be careful when IR testing on an installation with one of these fitted as you could end up borking it if you spike 500V DC from your tester down the line. You either need to disconnect it, avoid it or set your tester at 250v DC before any probe poking takes place.

When a surge occurs, the SY2-D is supposed to operate within 25 nanoseconds and self-reset afterwards, that is unless the surge was so great that the arse has been totally blown out of the thing, in which case the visual indicator should show red to inform you a replacement module is required. The surge module can itself be removed from the connection housing and a replacement unit slotted back into place which hopefully saves time out on site.

When it comes to costs, you can order directly from surgedevices.co.uk, and at the time of writing the list price is £64.00 plus VAT and shipping, although shipping is argueably a bit steep at £12.50 plus VAT for Parcelforce Express 24, unless you order four or more units in which case I believe delivery is free. I understand trade prices are available, and the company says these products can be sourced through Eddies, Rexel, CEF and Denmans, although you might have to push your local account manager to physically stock them, at least until they figure out that 18th Edition means we mere punters actually need to get our hands, and our screwdrivers, onto these things.

I notice Denmans now carry a Lewden branded item, model SRG1VCU which appears to be separated at birth from my SY2-D.


Surge Protection Devices Ltd. did tell me that they branded for Lewden, so there you go, it’s a rebadge, presumably with the same internal gubbins, and by gubbins, I mean a heavily insulated varistor at a guess. At time of writing, this part code doesn’t show up under a search on the ‘new and improved’ Denmans website, but if you’re after one of these then SRG1VCU or code 1078393 should allow the counter bod at Denmans to source them for you and I know my local branch have already started keeping them available off-the-shelf.

So, here’s the bottom line. If you’re on a new commercial installation, chances are you’ll be required to fit an SPD, especially if it’s in a non-urban location, that is unless you know the numbers for that daft risk formula and the calculation gets you out of it.

If working domestically on a rewire, new build single dwelling or a CU change, then offer it to your client, but it’ll cost them about an extra hundred quid just for the parts by the time VAT and any shipping is added on. Is that a lot to fork out? Well, it depends on how many high-end appliances they have, how flush they are and whether they live somewhere more prone to thunderstorm activity. A client’s insurers may also require surge protection to be fitted, especially if a previous claim has been settled following damage to electrical equipment by a direct or indirect lightning strike. They may pay out once, but they won’t again as lightning does strike in the same place twice and they’ll want their clients to take measures to protect themselves from it.

Prices will hopefully come down with economies of scale, and it may be the case that CU manufacturers themselves get on board and start including these devices alongside the main switch in their stock enclosures. We’ll see I guess. In the meantime, devices such as the SY2-D make the business of surge protection less onerous as this, in particular, is a rather neat and low-cost solution.

At the end of the day, justifying the cost for these things is the same as when people ask me if it’s worth them fitting wired interlinked smoke alarms: It’s worth that cost if the one time you need them, they are there. Better to have and not need than need and not have. Is a hundred quid really too much to ask when it comes to protecting your entire electrical installation from surges? I guarantee that if you say yes to that, then next summer when the lightning storms draw in on a humid day, you’ll be in two minds as to whether that was the right call.