After my stint at ElectroneX 2014 on behalf of element14, I was rewarded with a gift voucher to spend with them. That’s not a problem, as there’s always something to buy. In the spirit of a bargain-hunting uni student, I decided to chase some specials on test equipment that they had published in the Connect magazine, as test equipment can always come in handy. It also happens that, as a student with a student account with them, I get a further 10% off which helps stretch it just that little bit further.
In terms of test equipment, I’ve got one of most sorts of basic equipment, so I had to think a little more laterally. In this case, a special on a Tenma 72-9400 Insulation Resistance Tester caught my eye. After all, it’s a basic unit for AU$91.99, which is the cheapest I’ve seen one of these, and students get another 10% off, but have to add 10% GST, so it ends up at a similar price (i.e. 0.9*1.1=0.99).
People in the industry would probably know these by their trademarked name, a Megger, or their technical name – a megaohmmeter. These units produce high voltages (1000V DC) in order to measure the small leakages that pass through insulation. As insulation is designed to be almost completely non-conductive (i.e. high resistance), a high voltage is needed to measure the resistance in the insulation which is normally in the hundreds of megaohms to several gigaohms.
If you’ve ever seen the safety test tags on electrical equipment in workplaces, then you’ve witnessed one of the things a megger is good for – testing of appliances for safety. Insulation degrades over time, and due to transient overstress, and appliances with compromised insulation are at high risk of endangering users, so by testing it with a current limited high voltage DC source, it is possible to determine the health of the insulation without causing further damage.
Of course, electrical test and tagging have moved beyond the basic megger to more complicated appliance testers, especially as electronic controlled devices cannot be properly energized by a basic megger and thus can’t be fully tested, but it’s still worthwhile to have one around if even to conduct basic tests for your own interest and safety … and to possibly charge up capacitors and maybe even blow them up … okay, maybe not.
Tenma instruments are a little bit of a mixed kettle of fish. It’s a brand which is rarely carried outside of element14 and seems to provide a “value oriented” selection of equipment. The unit itself comes in a colour cardboard box, with the rear showing that Tenma is actually an Ohio, USA based company, and a spot for a UPC/EAN barcode that’s not printed on. Opening the box reveals a tightly packed carry bag.
Included inside is a set of eight Vinnic alkaline AA batteries (heh, batteries are included) which was a nice touch, a neck carry strap, as well as a set of test leads, probe and clip adapters. A very crinkled basic manual is included as well.
The unit itself is a solidly built brick, although slightly hollow due to the hard plastic shell. It comes with the yellow plastic cover clipped over the top of the unit, protecting the LCD display and controls.
The rear of the unit features the screwed-down battery door which needs to be removed to fit the eight AA batteries. The serial number is on the back of the device – and the label itself provides a clue of the manufacturer. As I’m no stranger to basic cheap test equipment, the serial number labelling looks like one from an old Uni-Trend multimeter I have, possessing the same 3-numbers in a different size at the beginning.
A quick search at the Uni-Trend website appears to show that this unit is identical to the Uni-Trend UT-501, so I was right! I will also note that the more advanced Tenma 74-9405 actually looks identical to the Uni-Trend UT-512 units.
The batteries fit snugly inside, with a battery door design that helps to minimize the impact of vibrations.
Removing the front cover of the unit shows the main interface. A single digit-style LCD display the results, with a twist-range dial that controls the measurement being selected. It is capable of being a DC and AC volt-meter, and performing insulation resistance tests at 250v, 500v and 1000v DC. The resistance measured is up to 2Gohms.
Further to that, there are buttons for data hold and for the backlight, which shines a white-blue.
Ominously, there is a button labelled test, and is of a latching type. Pressing it enables the high voltage outputs, and causes the button to light-up red. This reminds you that hazardous voltages are at the outputs.
The connections are made to the shrouded banana connectors on the front panel, and protrude outwards from the top. As a result, it’s not possible to pack it away with the cover on, without disconnecting the leads. The centre two terminals are used for voltmeter operations, with the outside two connections used for the megaohmmeter functions.
It’s a pretty basic unit, so there’s not much that can go wrong. Using it and interpreting the results can be a bit of an art, but there are some references and standards. In general, for mains appliances here, we should be testing at 500v DC unless there are MOVs in which case it can be tested at 250v DC, and resistance should be above 1Mohm, with anything less being suspect.
It seems that this unit beeps when the insulation resistance is below range or the output is shorted.
For regular appliances, testing should be done from earth to active and earth to neutral with the positive applied to earth (preferably). Furthermore, resistance to all exposed metal elements should be tested as well. For cables, tests should be done between each and every pair. The tester should be energized for a minute before the readings are taken. Changes in temperature and humidity can affect the readings as well.
Double insulated devices pose a little more of a challenge – testing should be done from each pole to the device’s enclosure – this probably is best done with a mesh or braid wrapped around the device.
Testing shouldn’t be done across active-neutral as the high voltage DC may cause damage to the device. Basic meggers such as this don’t allow for the device to be energized, so some electronically controlled devices cannot be fully tested as they can’t fully powered up without applying mains. More sophisticated (and expensive) appliance testers are needed for these.
Operating the unit is as simple as ensuring your test device is free of any charge, connecting it to the appropriate terminals in the appropriate manner, twisting the dial selector to the voltage required, pushing the test button, waiting the appropriate amount of time and reading the result off the screen.
I decided to give a few things around the house a test – many devices read over-range, which mean that their insulation is in tip-top shape, but my basic soldering iron measured 200Mohms … not dangerous but it shows that it’s possible to get a reading from it.
It’s a basic megger at a basic price. It’s a rebadged Uni-Trend UT-501 (aka Uni-T) unit. It does what it says on the tin, and it feels decently sturdy. It’s not something many people will want or need, but it’s a handy tool if you like to salvage stuff, do repairs or want some reassurance as to the safety of some mains devices.