Mains extension reel tester

Hi guys.

I'm about to start building a small box that will test a mains extension reel such as this type

My box will have a male 3 pin plug and female 3 pin socket coming out of it on fly leads that I can plug the extension reel into and plug in to the extension reel socket (thus creating a long loop)

The reels will not be plugged in to the mains, I simply wish to test for earth to earth/neutral to neutral/live to live continuity. No crosses or shorts.

I intend on red led indication of a fail and 16x2 lcd to output what type of fail was detected, earth to live short or no earth continuity etc.
A green led will indicate correct wiring with lcd simply saying test pass.

I've read nearly every thread on here about continuity testing with arduino and building cable testers. I've played with the method of one pin input pulled low with 10k resistor and other pin being a high output and then doing a digitalRead of the input pin, I understand this basic method bit had limited success with it, even with a 10k pull down resistor I seemed to get varying results (but that's for another post)

Main point of this post is checking if this project is feasible for arduino?
Will the long length of the reel be too much for the input high output low type of method?
What about if start test button switched a relay to put 5v with a higher current allong the reel to be detected by the input pin?

Any thoughts would be greatly appreciated


I'm pretty sure you can do this.

Look at THIS PAGE for the concept.

You would connect each of the 6 ends to an Arduino I/O pin through a 220 (or so) resistor. Also connect each of the 6 ends to ground through a large (say 270K ) resistor so they "Float Low". You would connect the 6 ends directly to the 6 analog inputs of the Arduino.

Think of the circuits you can create. Things like:

Switch both ends of wire 1 to High (5V), then test that the other 4 "ends" read close to 0 V (Tests for short between wire 1 and wires 2 and 3) Repeat for other combinations.

Switch wire1 A resistor to High and Wire 1 B resistor to Low. Read the voltage (you are reading between 2 220 ohm resistors as a voltage divider). (should be close to 2.5V if wire is intact).

Figure out and test fail patterns and write results to LCD. If there is a fault (Open/Short/Miswired) you will know what to fix

UPDATE: Per following posts, probably then test a "Good" Cable under a maximum load for a minute.

Make sense??

Let us know how it goes..

With 10k pullups and digital input, you could demo a toy tester for school, but it might not suffice for use by electricians because of the following failure mode : suppose that a 13 Amp rated live conductor has been cut almost through and is just about joined by the last remaining strand of copper. It will pass continuity test with a multimeter or with a a 10kOhm arduino test but immediately fail open-circuit if used. There are even worse partial cuts which could get hot when used.

I somewhat prefer the idea of using a relay (and a big separate 5V or 1.5V current supply) in order to measure the cable resistance with a couple of Amps going through. You’d still need to set up an op amp to get from a few mV at a few Amps to display on LCD of the mOhm of L,N,E and “PASS”. To beware of; different manufacturers use different grades of cable. I even saw one made in a (lets not name them) cheap overseas factory with no earth lead at all.

Depending on what you're doing, I agree that you may need to measure more than proper connection & simple continuity. You can confirm the current carrying capability by measuring resistance (Ohm's Law).

You don't need to run high voltage or current through it, but you do need to measure resistance at low resistance values accurately, less than one Ohm.

You should be able to find some examples of how to measure resistance with the Arduino, but you might have to make some modifications for low-resistance.

If you are manufacturing these things, you probably do need to test then under real-world voltage & current conditions.

The reels will not be plugged in to the mains

Why not? It would be super-easy to plug into AC power and then use one of [u]these[/u] (that's the U.S. version) the other end.

Of course a short would blow the circuit breaker, but if that's a big concern you could wire light bulbs in series with the non-ground wires (between the power line and the test-outlet socket). That would prevent the circuit breaker from blowing and a short would simply light-up the bulbs. Or, you could add a "local" lower-current circuit breaker.

Thanks for the replies people. Appreciate your input.

To answer a couple of questions, the reels won't be live at the point this test will be run. The point of this box is because I want a quick basic way of testing if it's safe to energise the reel. A full commissioning test does take place on the units including earth bonding tests, r1 r2 tests, insulation tests and trip tests but this occurs at a later stage and someone has decided to use the reels before a full commision therefore I thought since a lot of people don't know how to use a multimeter to run a basic test, a small box that can detect major flaws and give the user a red or green light may prove useful if they must plug in before the full commissioning test.

So from reading your replies it looks like it may be a better idea to read the resistance from start point to end point rather than just trying to make an input go high?

Just asking…
Your tester has a male mains plug on one side.
if i was to plug that into mains, i hope your box remains power safe.

Saying it will never be connected is like saying children can’t play with matches!

I'm not sure how many of these leads you will be testing, and I understand they will be eventually PAT Tested.

Most PAT testing units do a very quick automated full test of extension leads.

Use your PAT Tester instead of the unit you want to build, if you need to check because

someone has decided to use the reels before a full commission

then they should not be allowed to use the reel before full testing.
They should be QUARANTINED until tested.

Tom.... :slight_smile:

Just asking…
Your tester has a male mains plug on one side.
if i was to plug that into mains, i hope your box remains power safe.

Saying it will never be connected is like saying children can’t play with matches!

This I have thought about and will have to find a solution too

Hi, The main advantage of doing a more complex (easy to use once built) test is if there is a fault (Open/Short/Miswired) you will know exactly what to fix. Make a tag the tester attaches to failed units showing the fault(s).

I assumed the original poster was talking about a manufacturing situation.

I would too would have concerns about even considering a simple go - no go type test.

Someone might use it as 'proof' that the mains lead is good to go and the only way you can really be sure is a decent visual and a proper PAT test.

And you guessed it, whoever built the simple tester would very likley be liable for any accidents\fatalites that occur becuse the manis lead had a fault which a simple tester would never be capable of finding.

Any business, or other organisation, with a Health and Safety and safety policy should never allow it.

You can buy simple mains plug\extension lead testers very cheaply. After (but only after) a full PAT test, you could justify using one of these as a quick daily check or before every use.

I do understand the concerns you are raising, as I said though, people have already been told to use them before being commissioned, a simple test with a digital multimeter should suffice but not everyone knows how to use or read one hence the simple red green light. And even a simple continuity test with a multimeter can't highlight every eventuality and blame could be placed with whoever uses the meter anyway.
The idea is that some muppet has decided people should be using these before testing, the people that are using them have no idea on how to use a meter, if this box can throw a red light rather than just hopeing the rcd trips in time then it may be better than nothing