In my test rig video, I describe the construction of... well... a test rig! One, indeed, that any budding electrical inspector/tester can construct for themselves and then go to town on with their multifunction tester and clamp meter - observing safety of course! Here then is the part listing one would need in order to order in all the parts!

As already stated in the video presentation, the purpose of the portable test rig is to allow those getting to grips with inspection & testing to build themselves an environment where they can put the theory into practice. It is assumed anyone going to the effort and expense of such an endeavour is already well versed with electrical theory, installation, safe isolation and the requirements of GS38, and they are armed with the relevant qualifications and experience to know what they're doing to keep themselves and those around them safe. If not, then don't bother building the bloody thing; I cover how it works and what it shows in my video.

If undertaking the build, check further down the page for any errors that have been corrected since publication. The order codes and prices for City Electrical Factors are given here as well as some further information. CEF are not sponsors, nor are they necessarily the cheapest supplier in town, but they are perhaps the closest to most towns in the UK and so offer a good one-stop-shop for many of us, at least for all but two of the component parts. You may feel free to shop around of course. The prices given are per unit, inclusive of VAT and correct at the time of writing in January 2023, according to the CEF website. Before I get too many comments about pricing, I paid less for most of my parts through my CEF credit account, so consider these as worst case numbers. If you have (or know someone) with a credit account, you may find you're charged much less than the public list price.


Component   Qty   Description/comment   Stock code   List price (each)
Consumer unit   1   British General garage CU, with 40A RCD, 6A MCB, 32A MCB   2091-4075   £49.14
MCB   1   B16 MCB for the radial socket (a B20 could also be used)   1037-0272   £3.78
29mm pattress   3   Click Scolmore PVC surface box (two 20mm knockouts)   0669-2443   £2.82
47mm pattress   4   PVC surface box1   2704-6117   £2.30
1g socket   2   Hager Sollysta single-gang outlet   1088-1802   £2.42
1g USB socket   1   MK single-gang USB charger outlet   2653-3213   £11.00
1g two-way switch   2   Hager Sollysta single-gang two-way switch   1088-1748   £1.52
1g intermediate switch   1   Hager Sollysta intermediate switch   1088-1760   £9.64
Conduit angle box   1   Only available in red; remove the rubber inserts2   0049-8046   £9.12
Batten lampholder   1   Ensure it has loop terminals   0050-8273   £2.83
IP enclosure   1   238x190x903   1868-7349   £35.70
3.5 x 12 (8 x 1/2") screws   1pk   Pack of 200   0036-4330   £2.61
2-core 1.5mm flex   1rl   50m reel (not sold by the metre at CEF)4   0006-0334   £49.80
Plugtop   1   Ensure a 13A fuse is fitted4   0108-2893   £1.15
Tail gland   1   A 32mm stuffing gland could be used instead   2017-2943   £4.62
10mm 6181Y tail, brown   1m   Minimum length supplied by CEF   1873-6855   £2.88
10mm 6181Y tail, blue   1m   Minimum length supplied by CEF   1873-6852   £2.88
10mm 6491X green/yellow   1m   Minimum length supplied by CEF   0107-3254   £1.64
Click Mode 20A isolator   1   Any 20/32/45 or 50A double-pole isolator can be used. A neon indicator is recommended   0651-8485   £5.02
Sigma safety terminal   1   Any connector that can hold the 10mm cabling may be employed   1336-9615   £1.86
2.5mm 6491X brown   1rl   100m reel (not sold by the metre at CEF)5   0005-9893   £35.70
2.5mm 6491X blue   1rl   100m reel (not sold by the metre at CEF)5   0005-9884   £35.70
1.5mm 6491X brown   1rl   100m reel (not sold by the metre at CEF)5   0005-9815   £22.50
1.5mm 6491X blue   1rl   100m reel (not sold by the metre at CEF)5   0005-9806   £22.50
1.5mm 6491X green/yellow   1rl   100m reel (not sold by the metre at CEF)5   0005-9821   £22.50
32mm stuffing gland   1pk   For connection of the tails/earthing to the isolator pattress6   0070-2577   £20.82
16mm stuffing gland   1pk   For the supply flex entry (a 20mm gland or alternative solution may suffice)6   0070-2523   £8.22
4 x 12 brass screws   1pk   Supplied as a pack of 100, only two are needed for the lampholder!7   0018-2518   £12.80
Ideal Spliceline connector   1pk   Supplied as a pack of ten8   1574-6503   £2.88


1 The order code for the 47mm PVC pattress is correct, but it is not listed on the CEF website at the time of writing, nor are any alternatives. This code and price was obtained from my local branch who hold ready stock.

2 The conduit angle box is only available in red to my knowledge. If that messes with your feng shui, you can order in a conduit angle box in white, code 0008-8348 at £2.10, along with some PVC conduit, code 0009-1240 at £2.66 for 3 metres, and two female conduit adaptors, 0008-9959 at £0.76/each. These parts will perform the same task as the single angle box given above, albeit with a load of PVC conduit left over.

3 The 238x190x90 plastic enclosure allows each circuit to each have eight metres of wiring. I wouldn't recommend running any circuit with less, otherwise the readings obtained would be very low. If you want one or more circuits to have longer lengths, then a larger enclosure would be needed. Alternatively, two enclosures could be employed, one for the ring circuit and one for both the radial socket and lighting (assuming you want to run these together to observe the capacitive effect).

4 The 2-core flex and plugtop for the supply can be obtained by chopping them off an old appliance to save having to buy a whole 50m reel, but if upcycling such a flex, ensure a 13A fuse is fitted in the plugtop if you'll be playing with high current loads like heaters when undertaking current measurements. Some DIY stores sell flex by the metre or in shorter lengths of ten metres.

5 The 6491X conduit cabling used for the circuit wiring is, unfortunately, only sold in 100m reels at CEF. If you find somewhere that will supply it by the metre, order in more than the eight metres I allotted for each circuit's supply. Extra will be needed for the light switch wiring and to allow for any mistakes that may be made, or alterations you may wish to apply later. If buying from some back alley inernet souk such as eBay, make sure it's genuine copper cable and not some tinned forgery, otherwise your test numbers, like the wire, will be garbage.

6 Only two stuffing glands are used, both being on the isolator pattress. The 32mm gland is for the tails and earthing, the 16mm gland for the incoming supply cable. The CEF website retails each in packs of ten, although they can be obtained as singles in store.

7 Only two M4 bolts are needed in order to secure the lampholder to the conduit angle box. CEF sell them as a pack of a hundred, however a DIY store would likely be able to supply a smaller quantity.

8 The Ideal Spliceline connector is listed only because the video mentions that's how I terminated the CPC on the lower light switch, but any connector block could be used.

The following components depend on your preference for connecting the pattress accessories together, assuming you don't want to use conduit with female adaptors. You may use one of the following solutions exclusively, mix & match them, or do something else entirely:

If you want to use metal 20mm couplers, you'll need five along with ten brass bushes. These are best avoided between plastic accessories as they won't be earthed and single insulation wiring will be passing through them. If you want to use the compression glands, you'll need five (remember to remove the rubber bush inserts) plus five 20mm locknuts. If you want to use female couplers with bell mouth bushes (PHWOAR!), you'll need five of each.


Component   Qty   Stock code   List price (each)
20mm metal coupler and   5   0007-2952   £0.38
20mm brass bush   10   0007-2493   £0.49
20mm plastic compression gland and   5   0008-8645   £1.54
20mm locknut   5   0008-8744   £0.55
20mm female conduit adaptor and   5   0008-9959   £0.76
20mm bell mouth bush   5   0008-9764   £0.43

The only two parts you won't find at the likes of CEF are the backboard and resistor. For the former, use any bit o' board you can lay your hands on, be it MDF, chipboard, Contiboard or whatever. I mounted the IP enclosure on the rear of my board to minimise the overall size of the thing, but if you have a larger board or intend to install it all onto a wall, then the IP enclosure can be on the same side and adjacent to the CU. If that's the case, additional couplers may be required to pass the wiring from the CU to the enclosure.

Resistors may be obtained from the likes of Amazon, CPC, Farnell and RS. You may find a pack of ten will have to be procured and that there is a minimum order and/or delivery surcharge on such a low cost item. I used a 1R2 quarter-Watt resistor, but other values up to ten Ohms may be required for experimentation purposes. Always connect the resistor between neutral and earth, never between line/neutral or line/earth as it will explode.

My construction requires three sizes of holesaw: 32mm each for the tail gland, the stuffing gland into the isolator pattress and the (recommended) size between the rear of the CU through the board into the back of the IP box. A 16mm hole was drilled into the isolator pattress for the supply cable, and 20mm holes were drilled between the consumer unit and accessories.

If you'd prefer to use other brands of accessory, you go nuts. Personally, I'd avoid the cheap stuff that the likes of Screwfix and Toolstation punt as their in-house brands because while they may be good enough to be wired onto a wall and left in place, I can't speak for them in this kind of environment where terminals may be regularly twiddled. Technically, there's no advantage to the two-way switching plus intermediate - that's only there to give the thing more functionality and adds nothing to the testing and faultfinding aspect of it other than the R1+R2 being slightly higher on the switched side of the lighting circuit as opposed to the loop. In my video, a value of 0.21 Ohm was recorded on the loop, 0.24 Ohm on the switched side.

The incoming isolator can be a 20/32/45 or 50A white-plate accessory. The order code above is for a 20A unit with neon indicator. I recommend having the indicator so a visual warning is provided when the rig is energised. If you prefer, a rotary isolator or similar may be used.

If built, this item must be used with caution and only energised when under controlled and supervised conditions for as long as it needs to be to undertake testing. It should not be used to routinely power the electric radiator under your desk like some kind of glorified extension lead as it has no connection back to the supply earth!

Always ensure isolation before making any modifications or changes, when moving or placing the resistor and when undertaking dead testing. I shouldn't have to say that; if you don't already know it, then don't undertake this project. I know I keep making the same point, but just because I showed how I happened to build one of these things and what tests I could undertake on it doesn't mean it's safe for anyone else to give it a whirl. If you need to brush up on GS38, a free copy can be obtained from HSE here. For safe isolation, I have yet another video with a much more sober haircut from back in the day...

Errors, omissions and additions!

Anything that's mistaken or missed out of these presentations, or answers to common questions that crop up, if any, will appear here. I'll keep this page up-to-date, so check back if you either intend to construct one of these monstrosities or if you've taken my (possibly incorrect) word as gospel and are about to sit something like the 2391 exam!

Chris Shortt on YouTube made the good point that insulated accessories are better than metalclad. A portable unit such as this could see a lost neutral either through damage to the supply flex or with the wire popping out of the incoming isolator or Sigma terminal. In that eventuality, any earthed metalwork would go live as this is, effectively, an unearthed Class 1 appliance. It's important that before plugging this thing in to use it, the supply cable is first inspected. When going live, a voltage indicator to GS38 or a socket tester is used to verify line/neutral, line/earth and neutral/earth are all functioning as expected before physical touch contact is made with any metallic parts. With no supply earth in the incoming flex, there's no fault path to stop you from getting zapped. Again, construction and operation should not be attempted unless you're well versed with electrical installation and know how to handle such an apparatus.

For my rig, the flowchart below should always be followed when going live and before handling any exposed conductive parts or plugging in any Class 1 appliances...


Of course, if you know the installation your rig is to be used on, such as in my case with my own non-portable contraption located on the wall of my Twat Cave, then you can get around the whole unearthed Class 1 problem by running it off a non-RCD'd supply and using the supplier's earth in the way nature intended (i.e., by not joining it to neutral at the rig end). In that event, some experimentation will be required on the earth leakage resistor. My fixed rig requires a 10 Ohm resistor to operate in the same way as the portable unit and is driven by a 16A dedicated non-RCD circuit back at my CU.

Any Class 1 appliance you intend to connect to this thing for test purposes such as load measurements should be PAT tested first.

On another note, I've noticed there are small errors in two of the visual aids used in the presentation: Ring IR testing LN and LN-E. The ring line wires should look like they've been disconnected from the 32A MCB, however I've drawn them above the RCD like a dunce or some kind of fool.

YouTube user Ridleysss picked me up on a point in the Radial Socket IR chapter where I stated LIMitations must not be on an Electrical Installation Certificate (EIC) - "you put it in, you fully test it mate!" ...was something like the phrase I used. One exception to this, in my view, is a CU change because it's a hybrid job - a new board on old wiring. It's reasonable - expected even - for a periodic inspection to be performed on the existing circuits to ensure the consumer unit can be changed and the electrical safety signed off, but it's not reasonable to subject existing wiring to the rigours of initial verification. For a start, you won't have access to most of it because it'll be covered by the fabric of the building. In that respect, I'd perform an EICR first with IR testing at 250V and limited on L-N where it's impractical to remove all loads. When I author the EIC afterwards, those limitations would apply to that paperwork too as I wouldn't have any valid test data to enter onto the Schedule of Test Results. That's the only example of where I'd show LIM on an EIC schedule, and the reason for those limitations would be specifically mentioned on that paperwork.


A supporting PDF containing the testing visual aids used in this presentation along with the above flowchart is downloadable from here.

Current support file revision: 1.0
Dated: 15/01/23