Introduction
My old Kewtech voltage probe came to bits on me, so as it was no good to me anymore, I took the liberty of having a look inside it as detailed in an earlier blog.
I have spent a few days looking at a replacement and placed an order with Farnell, that arrived promptly the next day. Unfortunately, they didn't have the proving unit to go with this, so I purchased that from another well known online electronics supplier.
Unboxing
Thats right, I decided on a Fluke T110 voltage probe.
The T110 is an LED, voltage and continuity tester, with a built in rotary field indicator and pretty much offers the same functionality as my old Kewtech unit. I tend to prefer an LED display instead of an LCD as I find it much more clear to use for the testing for dead procedure. I also like the continuity function for testing fuses and contacts, as I do not carry a multimeter with me, and will rely on the voltage probe for basic fault finding if required.
Testing for dead
The main function I use a voltage probe for is for proving that a circuit is safe to work on, in my industry it is known as "testing for dead" and we call the process "prove-test-prove". You may also have heard it called "testing for voltage absence", a "live-dead-live" test or a "test-before-touch" test. The objective is the same, to provide as fool proof a method as possible to show that there is no voltage present on a circuit before work on it is started.
In the UK, working on electrical circuits is ultimately governed by the Electricity at Work Regulations (EAWR). The HSE also provide guidance documents for working safely on electrics, the two most helpful ones are;
HSG85 Electricity at Work - Safe Working Practices
GS38 Electrical Equipment for use by Electricians
These form the basis for the best practices of working on the high energy circuits throughout the plants I work on. They take the stance that a circuit should be treated as being live until it is proven to be dead. This makes the test for dead a necessity, which is, technically a live working activity, but can be justified under regulation 14 of EAWR.
The process is very simple.
1) Test your voltage tester against a known source of a comparable voltage level to the circuit to be worked on
2) If the voltage tester fails, then stop and find out why. If it works then proceed with testing the circuit
3) Test all live conductors against one another and each live conductor to earth
4) Test the voltage tester again with the known source used in item 1
5) If the voltage tester fails, then stop and find out why. Do not start work on the circuit
A neutral line is treated as a live conductor, so a single phase circuit has 3 tests; Live to Neutral, Live to Earth and Neutral to Earth
A three phase circuit will have 6 tests; L1 to L2, L2 to L3, L3 to L1, L1 to Earth, L2 to Earth and L3 to Earth
A three phase 4 wire circuit has 10 tests; L1 to L2, L2 to L3, L3 to L1, L1 to Neutral, L2 to Neutral, L3 to Neutral, L1 to Earth, L2 to Earth, L3 to Earth and Neutral to Earth
All potential sources into the circuit should be tested. The incoming mains is the obvious one. However, outgoing circuits can feed back in to the circuit due to borrowed neutral faults, or feedback from automatic changeover supplies. Long outgoing cables can retain a stored charge or there may be a parallel conductor that induces a voltage into the isolated cable. This is where the electrician needs to take time and think about what they are working on and understand where the potential hazards are.
Types of Testers
Under the guidelines from the HSE, the use of single voltage probes, non-contact probes and multimeters is frowned upon for use with the test for dead procedure.
Single voltage probes utilise the operator as part of the circuit. Their design and construction does not have safety as its first principle and the short-circuit failure of a single component within them can expose the user to an electric shock risk. They are therefore, considered high risk for testing on high energy circuits and are banned from all of the sites that I work on. Although they do make a great terminal screwdriver.
Non-contact probes are not regarded as reliable enough for proving that a circuit is dead and safe to work on. Their operation is intermittent and can be dependent upon where the probe is placed in relation to the apparatus. However, they are in use with some companies as an extra test as testing with a two probe meter requires the earthing and neutral systems to be intact. In some installations, where there is a risk of copper theft or broken neutrals, an additional test with a non-contact probe can provide extra assurance.
They are also good for initial investigation to see if voltages are present, but this is a different concept to testing for dead. Again, they are banned from all of the sites that I work on.
Multi-meters are a more complex instrument to utilise and incidents have occurred with people having the meter set to an incorrect range or the leads plugged into the current terminals by accident. On the high energy circuits that I work on, the latter can lead to a serious life-changing injury.
There are a number of videos on Youtube, showing multi-meter failures, the picture above is taken from one that shows the current terminal being overloaded. If you haven't seen the full video; Multimeter Failure
Some improvements have been made with regard to using multi-meters with GS38 approved test leads housing 500mA HRC fuses and less than 4mm of exposed probe tips. Other improvements have been made by manufacturers such as Gossen MetraWatt, that provide a shutter built into their MetraHit range of multi-meters to blank off the current terminal when the meter is set to read voltages and vice-versa. An alternative to this is a multi-meter without a current range such as the Fluke 114 unit. This also has a low impedance function with automatic AC / DV voltage detection to reduce the effect of stray capacitive or induced voltages that can give the impression that the circuit is still live.
However, when I am testing for dead, I am not interested in a specific voltage reading, all I need to know is what range it is in.
Multi-meters are not banned from the sites that I work on, they are very much needed by control and instrumentation technicians and the electrical discipline when protection relay and transducer testing is carried out. Their use for testing for dead is not permitted though. However, improvements such as those offered by Fluke and Gossen MetraWatt may lead this policy to be reviewed.
There are installed absence of voltage testers, the VeriSafe from Panduit, that was road tested for element 14, another one is the Grace Safeside. Whilst these can undoubtedly improve the safety aspects associated with a testing for dead procedure, for me, they do not fully replace it, as they require assumptions to be made regarding the installation.
Why the chosen tester
I have had a number of voltage probes and proving units over the years. When I first started, Drummond MTL7 / MTL9c and Martindale VI 13700 testers were common place. These were incandescent and neon based units respectively, only offering voltage detection between 50 and 500V AC/DC. But they did not require batteries to operate. Both of these units have been modernised, now utilising LED indicators and GS38 compliant test leads and still retain the battery-less operation.
For me, the lack of a continuity function reduces the usability of these older style probes. There are numerous manufacturers today offering combination testers that offer voltage detection, continuity tests, RCD trip tests, phase rotation and even work lights built into the probes. These are naturally battery operated devices, but some will still measure voltage without the batteries, the Fluke T110 doesn't quite do this, it will light the hazard light to show a voltage is present, but will not indicate the voltage level. This is the same functionality as my old Kewtech unit and one of my reasons for opting for the T110.
The T110 also has a load function that can be manually activated to increase the current drawn by the meter from 1mA to 30mA at 230V. The primary use of this is as a quick test for an RCD unit. It is also used to discharge stored or induced voltages, something I come across a lot on the type of sites that I work on.
It comes with swappable probe tips between a GS38 shroud and a 4mm diameter extension. Undoubtedly, mine will spend most of its time with the GS38 shrouds attached, but the 4mm extensions are supplied to make testing socket outlets easier, something that usually ends up bending probe tips if not done carefully. Very cleverly, these shrouds and tips can be stored within the tip protecter supplied with the instrument, so they will always be with it and hopefully will not get lost. If they do, spares can be purchased from Fluke, whereas with some other manufacturers, a new voltage tester would have to be purchased.
The tip protector has also been designed to aid with the removal and fitting of the tip adapters. The GS38 shrouds are a snap fit onto the probes, so pushing the probe into the shroud will snap it in place and then the shroud can be pulled out of the tip protector fitted. To remove the shroud, the probe is pushed back into the tip protector and the tip protector can be used to grip the shroud whilst pulling out the probe. The 4mm adapters are a screw fit, so are a little easier to fit and remove, they are removed from the tip protector by inserting the probe tip and twisting a quarter turn to engage the threads. The probe can then be withdrawn with the adapter fitted and tightened up using fingers. The adapters are removed by hand and then just pushed into the tip protector for storage.
The tip protector also has an earth prong built into it that can be inserted into the earth socket of a BS1363 outlet to lower the safety shutters on the live and neutral sockets and allow the tester access to measure the voltage. Whilst this adds to the versatility, for me, it would need to be used with caution, especially in a domestic environment, where a child may see this being done and encourage them to poke other items into the socket. In these circumstances, my preference is to use a proper socket function tester, a 3 pin adapter or a plug wired with a lead and some 4mm safety sockets.
Other functionality I will find useful that the T110 provides is a work light on the probe, that can be turned on manually and also the ability to turn of the audible notification of voltage presence. This is useful when the instrument is being checked on the bench.
With all this functionality provided, I felt the T110 offered good value for an instrument from a reputable manufacturer for a reasonable cost.
Proving unit options
I still have my old Kewtech proving unit, but I thought I would look at the offerings from Fluke to match the voltage probe. Two units are available from Fluke as per the picture below.
The first unit is the basic PRV240PRV240 that offers a 240V AC or DC output The second unit the PRV240FS offers the same 240V AC or DC output along with the added functionality of a Field Sense output I was curious about the Field Sense to see if it would work with the non-contact pens and the Vsense function of the Keysight multimeters so I opted to purchase one of these
The output on the PRV240FS is changed using a 3 position selector switch on the front of the unit. The unit remains dead until one of the contacts is pressed down with a meter probe or the finger ground contact is pressed down when using the Field Sense option. I could get the two non-contact pens I had available to operate with the Field Sense. The Vsense function on the Keysight multi-meter could also be tested in the same manner.
In terms of using either of the Fluke proving units with a voltage probe I become a little wary as the output of the proving unit is limited to 240V AC or DC This is fine for testing on single phase apparatus but when working on three phase I prefer a proving unit that lights up at least the 400V LED and preferably all of the LEDs I also found that the output of the PRV240PRV240 must be current limited as it could not light up the Drummond MTL20 voltage tester
Testing the Voltage Probe
The tolerance for the ranges on a voltage probe are quite wide, this is an instrument with a specific purpose of testing for voltage presence / absence, so ultimate accuracy isn't required. The tolerance bands are set out in IEC 61243-3 and I tested the Fluke voltage probe against these tolerances using a three phase injection test set that can be reconfigured to provide a single phase 600V output.
The table of test results can be seen below. Both the AC and DC pickup values can be seen to fall well within the tolerance bands for the standard.
The continuity function of the voltage probe can be tested against a resistance decade box. The specified pickup value from Fluke is 400kOhms but under the standard this has a +50% tolerance to it. So the probe can see continuity as anything up to 600KOhms. Tested, this particular probe saw continuity up to 573kOhms.
Whilst testing the continuity function using the decade box, I initially used the 4mm probe tip adapters thinking that the would fit straight into 4mm safety sockets. Unfortunately they did not. To an extent, they did fit into the Kewtech socket adapter but not all of the way. When measured, the adapters were found to have a diameter of 4.1mm, explaining why they did not fit.
My Kewtech voltage probe and proving unit were kept in the same tool bag as some of my hand tools and that may have been one of the reasons for it coming out in two separate parts. For the T110, I decided to purchase the Fluke C150 soft case which is recommended for it. Unfortunately, it is a bit of a squeeze to get both the probe and the proving unit into the case, they will just fit, but it will probably shorten the life of the case. Martindale do offer a bespoke case for their testers with a separate pouch for the proving unit, unfortunately, the Fluke T110 is just a little to long to go into this case, otherwise it would have been a good alternative.
Safe working practices
I will close out with a brief overview of safe isolation methods. This should actually be carried out prior to testing for dead, so a brief procedure could be;
1) Identify apparatus and workscope
2) Identify points of supply
3) Isolate and secure points of supply
4) Test for dead
5) Carry out work
6) Remove isolation and restore supplies
There could be many elements to this involving various authorised persons and permits to work for formal system on large sites to individual work parties on small domestic / commercial installations. The procedure and system need to match the risk to ensure that a robust isolation and work method match the risks of the work and do not increase the work burden.
The procedure used to secure an isolation is termed as isolated, locked and tagged in my company and the various points will generally be identified on a formal permit to work. On large plants, others applying isolations has the advantage of having multiple persons verify that the isolations are correct, a second set eyes to verify that nothing has been missed. More commonly you may have heard the procedure termed as 'LOTO' - Lock Out Tag Out.
There a e a number of devices available to secure isolation points depending upon the particular device to be locked off. From small devices for MCBs, through larger devices for main incomers or cable systems for multiple isolators or valves. Some common devices for electrical points can be seen below.
An important element is that most of the time the padlock should have a unique, individual key to prevent errors when removing isolations. I tend to prefer manufacturer specific devices when available as they tend to be more robust, but there are plenty of generic devices designed to fit multiple manufacturers apparatus. Larger main isolators can usually be locked off directly with a padlock, or a multi-hasp if more padlocks are required to issue multiple permits.
The generic devices I have for MCBs require a screwdriver to tighten down a retaining screw onto the MCB lever. A padlock is then put though the hole to guard access to the retaining screw. These devices are ok, but the MCB can be damaged if the retaining screw is over tightened. The thumbwheel padlock works in a similar manner, except that it does not require an extra locking device as all the other devices do.
Summary
The T110 voltage probe feels robust and comfortable to use. It has some interesting design features around the tip protector that will reduce the likelihood of loosing the tip adapters. The availability of replacement tips though is an added bonus. The proving units also has some good features, but I would prefer it to have a higher voltage output. I would also like to see Fluke offer a bespoke case that can house the tester and proving unit together, similar to the offer from Martindale.
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