 | |  | | | Megger’s DCM330 fork meter | |
Clamps for convenience
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| 02 Feb 2010
If you're an electrical contractor and you've got all the instruments you need for testing installations in line with the IET Wiring Regulations, why would you spend any more money on test equipment? The answer, as Keith Wilson explains, is that making a very modest investment in a clamp meter can save you a lot of time and trouble.
Clamp meters essentially measure current, so it might at first seem that they are of little use to a contractor who already has a multimeter or similar instrument with current ranges. Clamp meters, however, have one outstanding advantage over ordinary instruments – it's not necessary to break into the circuit to measure the current flowing in it. Instead, all that’s needed is to clamp the jaws of the instrument round the conductor and the current will be instantly displayed.
There are clearly many benefits to taking measurements in this way; it's faster, it's safer and it's less disruptive, as there is no need to isolate the circuit to allow the ammeter to be connected. In addition, clamp meters can be used to carry out tests that would otherwise be difficult to perform.
Earth leakagesFor example, some types are optimised for checking earth leakage and are invaluable in tracing faults that lead to nuisance tripping of RCDs, and for verifying that leakage currents are at a safe level in circuits that are not RCD protected.
To check the earth leakage in a single-phase circuit, one method is to clamp the meter round the live and neutral conductors simultaneously. The currents in the conductors should, of course, cancel out so that the instrument reads zero, but if there is any leakage to earth this leads to a difference in the phase and neutral currents, which can be read from the instrument display. Alternatively, the instrument can be clamped around the earth conductor so that the current flowing in this is measured directly. It should be noted, however, that this will not necessarily show the total leakage, as current may be finding its way to earth via routes other than the earth conductor.
An instrument that has been specifically designed for applications of this type is the 360 AC Leakage Clamp Meter from Fluke. This pocket-sized instrument is rugged and lightweight, and it can display leakage current as small as 1 microamp. It has a maximum current measuring capacity of 60 A, so it is also suitable for general purpose use in checking, for example, the loading on individual circuits in distribution boards. A special jaw design makes it easy to take measurements even in confined spaces and a data hold button is provided to retain the reading on the display after the clamp has been removed from the conductor.
High currentsFor users who are more interested in high currents than earth leakage, Seaward offers the Type L Plus clamp meter that can measure currents up to 2,000 A. It has an extended jaw size of 100mm and can, therefore, be used to check current flow through large conductors, including the porcelain fuse carriers often used in feeder pillars.
Fluke also offers clamp meters capable of measuring currents up to 2,000 A – the true-RMS reading 353 and 355 models. These have a special mode button for measuring in-rush currents, such as those associated with starting motors and switching inductive loads, and they incorporate low-pass filtering for more accurate measurements on circuits where high-frequency or switching noise is present. In addition to measuring current, the Fluke 355 also has facilities for measuring voltages up to 1000 V DC and 600 V AC, as well as resistance up to 400 kΩ.
The KEW5026R clamp meter from Kewtech is another instrument with multimeter capabilities and in addition, it offers the option of disabling the auto power off function. This may not sound very useful, but this instrument also has peak load and minimum/maximum functions. With auto power off disabled, therefore, it can be left to monitor a circuit continuously for several days, until its batteries run out.
This can be very useful for determining peak load without the need to use an expensive power analyser. A further benefit of the KEW5026R is that it reads true RMS, which means that it gives accurate readings even with heavily distorted waveforms.
Applience of scienceUseful as clamp meters are, most models do have one rather irritating limitation – they can’t easily be used to measure the current consumption of an appliance like a washing machine or a television. If the appliance has a two-core cable, clamping a meter round it will simply show the earth leakage, and with a three-core cable the reading, if any, is likely to be meaningless. Nevertheless, it is often very useful to quickly check the current used by an appliance, not least as an aid to identifying opportunities for energy savings.
Fortunately, Megger has come up with a solution in the form of its MMC850 multi-core clamp meter. This can be used to measure the current flowing in one-, two- or three-core cables of flat or round section, up to a maximum of 100 A. There’s no need to split multicore cables – the whole cable is simply clamped in place, and the current can be read immediately from the display, with a resolution of 0.1 A. The MMC850 works equally well with 50 Hz and 60 Hz supplies, and it has a CAT III 600 V safety rating.
Get hookedAs we’ve seen, clamp meters are easy to use because it’s not necessary to break the circuit to make the measurement. But what if you don’t even want the bother of operating a clamp? Well, if you’re working on overhead power lines, Seaward has just what you need – the Halo hook-on ammeter that can measure currents up to 2,000 A in conductors as large as 50 mm diameter working at voltages up to 36 kV! As its name suggests, this instrument incorporates a sensor in the shape of an open hook and, in use, it is simply hooked over the power line.
Of course, most contractors will be looking for an instrument that’s a little more at home with lower voltages and currents, so what about the Megger DCM330 fork multimeter? This has an open jaw, and all that’s necessary when taking a measurement is to place the cable in the jaw – no clamping is needed, which is very useful when working in a confined space.
The DCM330 measures current up to 200 A, and also incorporates an auto-ranging voltmeter covering 0.1 V to 1,000 V, resistance measuring facilities from 0.1 Ω to 20 MΩ, a continuity buzzer, a diode checker and a non-contact AC voltage detector, making it a very versatile instrument.
It’s easy to think of clamp meters as an unnecessary luxury but, as we’ve seen, they can be absolutely invaluable for making current measurements quickly, conveniently and safely. They are also an exceptionally useful diagnostic tool for use when investigating earth leakage problems and the more specialised types can readily tackle tasks for which it would be much more difficult and inconvenient to use any other type of instrument. It may, therefore, be well worthwhile investing in a clamp meter, if you don’t already have one – then you’ll quickly discover that clamps can be very convenient! | |
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