Pleased to meter

How many times have I heard the expression “look after your tools and your tools will look after you”? We all know it is true, so why do we all take our tools for granted and just abuse them till they let us down? Why do we just throw our tools in a box or a bag without due care and attention and without rhyme or reason?

To some extent I can live with the "throw your tools in a bag" merchants, until it comes to meters. Meters are like an extension of your arm, or to be more accurate they should be an extension of your brain – they are the one reliable reference point in a market of variants, or at least they should be. But how can we rely on the accuracy of our meters when they are thrown in with a bunch of hammers and screwdrivers and never checked? Go on, own up, how many of us change the batteries in our meters before the screen goes blank? I have one hand free, shall we start counting?

Looking after your meter and understanding what it tells you is perhaps the most important part of your job; fail that and you are quite possibly leaving behind you a trail of faults and problems and it is often others who have to pick up the pieces, so where are we going wrong?

What are we after?

Choosing your meter is the first step. All too often I see meters that have been bought on price and not on facilities. Let's put our cards on the table – a crap meter equals a crap engineer, so why do I see so many horrible little meters bought at the local pound shop? You are a professional engineer, and you deserve a professional meter – your meter is a fundamentally essential tool and it needs to be treated with the right mount of respect.

Your meter needs to have certain ranges and capacities and if you don't have them you cannot complete your job, so let’s start there. First you need to be able to read the mains supply, so a reasonable range of AC voltage should be there, ideally with a range that gives 230 volts somewhere around the middle. What are we looking for? For a start we are not looking to replace the more robust mains testing kit that we need for testing the fused spur and issuing the appropriate certificate of electrical soundness, but we are looking to be able to stick the meter probes on the mains terminals to check incoming supply and check that the voltage is what it should be and where it should be. The first time you use it is not the best time to find out that the 230v supply is off the end of the range of your cheap meter. By guessing what you think the meter has told you, you could find out the hard way that you are very wrong. It is the equivalent of using two pieces of wire with a croc clip on the end, attaching one crock clip to each ear and placing the other bare ends in the mains terminals – when your eyes cross and you fall on the floor dead as a doornail then you have found the mains. This is also the time NOT to use your broken test leads – you know – the ones where the insulated probe has fallen off the end and you have twisted the bare wires round your cheap non-insulated terminal screwdriver and have to place a good strong finger on the bare joint to get an accurate reading. It is NOT A GOOD IDEA to use these on the mains input. You get a lit up engineer and I am not talking about too much of the Christmas sherry.

Perhaps it is a good idea to check the "mains in" on every visit before we start fooling around in the panel. You won't be the first engineer to fall foul of some home DIY mains wiring at the fuse box. I know it is not conclusive but I always took time out (about 10 seconds worth) to give all metal panels a quick poke with a mains tester screwdriver, on the odd occasion that the screwdriver lit up I treated the panel with great respect until I had proved it safe.

Upon opening the panel a quick check should give us a 230-240v reading between the live and neutral and between live and earth; the reading between neutral and earth should be zero. If you don't get those sorts of readings and you are competent, then start checking for a mains problem; if you are not sure, call back the last "electrician" that was working on the system. If you are not sure and not competent shut the panel and write down your findings on the job sheet and get the customer to sign it. Make sure the customer knows there is a potential mains problem and to call in an electrician. Today more than ever it is essential to be able to protect yourself. With the changeover of mains wiring colour schemes there is always a possibility of mistakes being made and you finding a 415v crossed mains supply, so make sure your 230v range will cover 415v as well or has another higher range that you can step up to. There are no prizes for going home in a box!

Check the output

You will also need a lower AC range to check the output of the mains transformer to the panel. It should be somewhere between 15v and 22v, give or take a volt or two. The transformers don't often go wrong and we tend to take them very much for granted,but they can cause all manner of problems if they do, so the odd 15 seconds it takes to check the output is not really lost time – blind faith is not the best tool in your box. Ideally a range of 0-50v AC should be suitable for this but I see so many cheap meters that fail to give us that range at all, and the luckless engineer who is reading the transformer output on a 230v scale using a cheap meter is usually wasting his time.

A crap meter equals a crap engineer, so why do I see so many horrible meters from the pound shop?

Let’s go one step further down still. Do you ever test for induced AC voltage on the DC circuits? No? Well, try it some time. For this you will need to set your meter once again to a low range of AC volts and place your probes on the incoming earth terminal and any other terminal on the board. A bit of experimentation will show that you get the same AC volt reading on both 12vDC and 0vDC terminals. You may have to trawl up and down the ranges; I have taken readings as high as 47vAC and down to zero.

A few points to bear in mind here – one, most metal boxed panels now have their 0v strapped to earth for safety reasons. This usually cancels the induced volts reading. Two, today's modern panel can usually cope with these unwanted inputs so they are not critical but the battery will not like the AC and may well expire early, so it may be an idea to filter it out. Finally, if you are taking over a strange system or looking at second fixing a system where the cabling was done by others, the induced AC check can often give an indication of how close the 12v cables have been run to the mains cables. Just to sidetrack here, I often see the spare unused cores of a cable very neatly wound round the end of the cable like a multi coloured shirt cuff. It looks very neat and professional but the coil round the cable is a very effective amplifier of induced signals, and in the old transistor days was a guaranteed way to cause false alarms. If you want to do something useful with the spare cores, try strapping them to the incoming earth terminal (make sure you have a good earth first). It reduces the unwanted induced signal that the nice neat coil amplifies.

Back to the script – cheap meters are also often lacking in the current ranges as well. We need to be able to read the current supplied to all the relevant bits of kit, and some of them are down to maybe 5 milliamps. Very few companies take current readings on individual detectors nowadays and it is their own loss. If a detector or any piece of electrical kit goes faulty it often shows up as a change in the current consumed. Taking the current readings at the commissioning stage is not meaningless – you are proving the installation by finding any faults, and recording a reference point that will help you find a fault later.

We also need to be able to read the current drawn by the whole system, both at rest and in alarm. These readings could be 800mA or more even on a small system, so to be on the safe side we need a range up to, shall we say, 2 Amps fused. Some meters have a poor range, maybe only up to 200 mA, and after that the fuse blows. Readings beyond that point have to be taken on the higher UN-FUSED range. You really should have a meter that gives a range or ranges up to 2 Amps fused, then we can take the right readings and find the right answers.

A spare fuse?

One thing I often find is that the engineer has no idea that there is a fuse inside, it is often blown and all readings are taken on the higher UN-fused range which may well not give us the accuracy we require. Which brings me to another point – why do the manufacturers of meters always have us taking the whole meter apart to change a fuse? How many of us carry a spare fuse or two for our meters? Perhaps we ought to.

Choosing the right meter is just a matter of common sense in looking for the right or at least usable ranges. It is also good common sense to look after your meter and not to throw it in with the rest of the tools. It is also worth looking at buying a meter with a good robust cover – the meter is not designed to be bounced on the floor from 30ft up a ladder, but it happens by accident so prepare for the worst and make sure your meter is well protected. Just before you reach for the pen to remind me, yes, I am aware of the new working at height regs, but I also know a lot of people are not yet aware of them, so I have used a little "industrial licence" in the way I write.

Good, so now you have found the right meter – how do you know it is accurate? Usually the answer is that the owner doesn't. "It was good when I bought it so it will be good now" is not the right answer. Ideally, you should have your meter checked or "calibrated" once a year (or so), but it is not always convenient to send your meter away for a fortnight every year to be calibrated, and it is not always viable to carry two meters. There are alternatives – you can have one calibrated meter back at the works and check all other meters against it. You can buy one good new meter every year, deem that to be accurate, and check the others against that one. You could just do comparison checks between several meters; if one is out it will show up.

Finally, if you are NSI approved your inspector will want to see recorded evidence of the checks and tests that you have done, properly signed by whoever did the testing. The aim is to confirm that you HAVE checked your meters and they ARE within your own acceptable tolerances. It is not the NSI laying down rules, it is only what you should be doing for your self. How else would you know your meter was accurate?