Continuity test after Ring Circuit

[Neptune]

After completing a new RFC or modifying an existing one, I will always test continuity across all 3 conductors at the Consumer unit. I use the continuity function on my Fluke 2 pole tester to do this. This is done whilst the circuit is dead.

Prior to this, I am very diligent with the conductors in each socket terminal and ensuring that I have the correct amount of cable length. I also screw the terminals tight again once the excess cable is bent in the back box and before I screw in the socket plate incase anything had come loose.

This means that when it comes to the continuity test, everything is okay. Else, I will revisit each socket, rectify and test again…

I know “proper” electricians will do R1, etc. tests using a low ohm meter. What am I missing without doing these tests and can any of these be done without buying the very expensive low ohm meters that electricians have?

I appreciate that the very simple response is that these test have to be done.

Thanks in advance.

 

[buzzlightyear]

The purpose of a low hom meter is to test a ring or other to see what values you have got.

 

[Neptune]

The purpose of a low hom meter is to test a ring or other to see what values you have got.

Thanks but can you elaborate on this please. What am I practically missing from my approach versus using this meter?
Also, are there any alternatives to the very expensive low ohms meters?

 

[westward10]

The short circuit current for low ohm resistance testing should be 200ma.

 

[Electro-tech]

Without R1+R2 ohm values, how will you know whether your overcurrent device will function under fault conditions, or have you chosen the correct one.

 

[Eddy Currents]

What am I missing without doing these tests

Basically you are confirming continuity of the ring and skipping all of the other tests which are required to confirm the circuit is safe to energise and use and will disconnect in the required times in the event of a fault

MFTs do all the required tests not just low ohm readings. R1+R2 will confirm earth at every point and also polarity. Values are recorded and checked against tables in the regulations to ensure they meet disconnection times but other tests like insulation resistance should be confirmed

Also how are you testing and recording values for a new RFC?

 

[Dartlec]

The two separate tests for a RFC that use a low resistance Ohmmeter are to check slightly different things.

The initial continuity one is to ensure that there is a "ring", which any continuity tester can show to some extent.

However, the point of the low resistance Ohmmeter is to get actual values for r1 (Line), rn (Neutral) and r2 (CPC)

The values for Line and Neutral should be consistent with the known or estimated length of the circuit, and also very similar to each other (<0.05 difference generally). That ensures that the ring is consistent with no strange loops. It also verifies that the terminations are satisfactory and not introducing resistance.

The CPC resistance should be higher, relative to the smaller size of the CPC compared to Line and Neutral (1.67ish with standard 2.5 twin and earth)

A 2 pole tester that shows continuity will give you no idea of how good the connection is, and wouldn't show up, for example, a poor loose connection on one leg of the ring.

That may be avoided if you are installing the circuit from scratch and know exactly where all the connections are, but on an existing circuit there may well be hidden junctions boxes, or connections that are not immediately visible.

Then there is R1 + R2 which is tested at each point, and is ensuring that the earth fault loop resistance is low enough that the installed protective device (mcb usually) will trip if an earth fault occurs. The tables give maximum readings for Zs, which is the combination of R1+R2 and the external earth reading Ze, though Zs can be tested directly too with a multifunction tester, or a separate earth fault loop tester.

So there is a reason behind the requirement to test and get proper readings, even before the final reason that a certificate when filled out needs the values to be inserted, to assist future electricians who might be troubleshooting the work.

These days, there are some older Multi Function Testers going quite cheaply on Ebay and the like. Something like the MFT1552 is almost bullet proof and often costs not much more than £200. That will still do every test you might need to do in a domestic setting and really is a sensible investment for anyone doing more than a one off job, even if it's just replacing the odd socket.

 

[buzzlightyear]

Using the low reading ohm meter, test between Line and Earth on every outlet on the ring circuit. The highest reading is the (R1+R2) for the circuit. The readings at each of the sockets wired on the ring should be very similar and the value should be according to this formula: (r1 +r2)/4.

 

[Neptune]

A 2 pole tester that shows continuity will give you no idea of how good the connection is, and wouldn't show up, for example, a poor loose connection on one leg of the ring.

That may be avoided if you are installing the circuit from scratch and know exactly where all the connections are, but on an existing circuit there may well be hidden junctions boxes, or connections that are not immediately visible.

With a new circuit - and perhaps naively - I know that each connection has been correctly inserted and the ring follows the correct format. The continuity test further validated that we have a “circuit”. I’ve assumed that is adequate.
Realistically, where are the additional tests likely to pull out issues in this scenario? I appreciate that existing RFC’s May have other lurking issues that such tests draw out.

Something like the MFT1552 is almost bullet proof and often costs not much more than £200. That will still do every test you might need to do in a domestic setting and really is a sensible investment for anyone doing more than a one off job, even if it's just replacing the odd socket.

Will this do the insulation resistance test too?
Also, do these devices need regular calibration and have to be sent off to a specialist for this purpose?

 

[timhoward]

Great answers already.
In short and sweet terms you are missing checking for
-loose connections
-enough fault current will flow to trip the protective device
-accurately checking ring end-to-ends and relative length of CPC
-you haven’t accidentally created a loop in the ring

I’d assume this means you also aren’t doing IR tests so you aren’t checking for damaged or snagged cables adequately either.

I’d also assume you aren’t able to test the mandatory RCD is working for the new sockets circuit.

I know it’s painful but you are at the point you need an MFT. You could buy cheap separate units but I’d say it’s better to get an old 1553
(Or maybe a Kewtech KT53 as it’s easy to use and a decent first tester)

 

[Dartlec]

Will this do the insulation resistance test too?
Also, do these devices need regular calibration and have to be sent off to a specialist for this purpose?

Yes any of the MFTs will also do Insulation Resistance as well as RCD testing of the sort needed in domestic (aside from car EV chargers, which is a different specialised segment.

Occasional third party calibration (usually is in the £40-60ish price range) is ideal, but not always necessary Something like a Calcard will allow ongoing verification of the results for continuity and insulation. Zs (Earth Loop) and RCD test can be checked against a known circuit to check for any drift over time.

To be honest with most MFTs, issues with reading errors are almost always down to the leads which are easy to replace.

 

[Dartlec]

With a new circuit - and perhaps naively - I know that each connection has been correctly inserted and the ring follows the correct format. The continuity test further validated that we have a “circuit”. I’ve assumed that is adequate.
Realistically, where are the additional tests likely to pull out issues in this scenario? I appreciate that existing RFC’s May have other lurking issues that such tests draw out.

If everything is installed perfectly from new then the tests are really just a way of proving that you've done the due diligence.

In practise, unless the cabling is all visible from start to finish, there is always the opportunity for things to not be what they seem, and the tests are designed to ensure that the installation is safe to energise/re-energise.

Even with a new install of cable there are multiple things that could go wrong:

Has the plasterer nicked a cable with his trowel, have the skirting board installers put a nail through a cable, has the home owner fitted a picture right where a cable runs down a wall (perhaps from the other side of the wall without realising), was the cable manufactured with a fault, or was it sold as genuine cable when it was actually cheap, non compliant cable.

None of those would necessarily prevent the installation from working, but they may well mean that it is dangerous to leave powered.

With an existing RFC then there are the hidden junction boxes under floors and cables that have been chewed by mice to consider, or perhaps a section of older rubber 50s cabling left in place that is starting to deteriorate.

The main benefit of a proper test instrument and doing the tests is that you know you've done the work safely and can prove it. The certificate is as much for your protection as it is for anything else. Should a fault develop a month later due to other work, being able to show that the readings were correct gives you a level of immunity from potential comebacks. Not to mention if something more serious was to happen, you could prove to any investigation that you have met your legal obligations.

 

[Neptune]

Thanks for all the responses. I am up for purchasing the meter and as someone said, I think I have now reached the stage where this will be necessary. I may have some more questions on the precise model that I am intending to purchase.

In the meantime, I have a ring main issue that you may be able to help me troubleshoot please. Here is some background:

• I installed two separate ring mains. They are both on the same RCD and wired into a 32amp MCB
• Both have continuity across all 3 conductors when tested with my 2 pole tester. As you know, I don't currently own a low ohm meter
• 1 of these ring mains works fine and we have been using it for a couple of weeks
• The other ring main, when energized, tested successfully using the plug in tester across all sockets but when I plug a bosch hammer drill (as an example), it trips the RCD. This happened in all of the sockets I tested.
• The neutral for this ring is on the terminal bar for the correct RCD
• As a further test, I wired (Live into the MCB on the other side and Neutrals into the associated terminal) the affected ring onto the other RCD (I have a dual RCD CU) and again socket tester is fine. It lights up to indicate that everything is okay but as soon as I plug in my drill (or a kettle and even a phone charger), it trips.
• I then reverted it back to the original RCD and same issue remains.
• Leaving these wires connected was causing the other circuits to misbehave. As a example, the heating thermostats were happily switched on but when we interacted with them, they were also causing the RCD to trip. When I unwired the affected Ring, the thermostats worked fine.

I am hoping the above provides some clues as to where the problem may lie and how I troubleshoot it. One approach may be to have a floating wire which returns to the consumer unit as one end of the ring and I connect this to each socket at a time. Starting with the first socket in the ring and testing this with the drill (or anything I suspect!) as I go along.

Over to the experts.

 

[timhoward]

The other ring main, when energized, tested successfully using the plug in tester across all sockets but when I plug a bosch hammer drill (as an example), it trips the RCD. This happened in all of the sockets I tested.

This is classic behaviour for higher a resistance fault or maybe a Neutral - Earth fault.
To the best of your ability you have proved that each conductor has end to end continuity.
A 2 pole tester will only report continuity of up to about 200 Kohms and will use a low test voltage to do it.
I'm assuming your 2 pole is NOT saying that there is continuity between any of the conductors?

To diagnose this further you urgently need a continuity and insulation resistance tester which can test using up to 1000V and measure from hundredths of an ohm to hundreds of megaohms.

Are you feeling lucky? This powers up, if it works it is an excellent tester that would transform your life!

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As to your fault, my best guess is a nail used for capping has missed and has damaged a cable. Or a screw at a socket has caught the Neutral.
Breaking down the ring, with an IR tester is the only way to proceed I'm afraid.

 

[timhoward]

Here's a set of separates, IR/Continuity, Loop, and RCD
The loop tester won't work with RCBOs but you can at least calculate Zs.

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