Megger

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Meggers test the insulation of appliances & wiring with 500-1000v.


Contents

Which Megger

A Megger can be bought when needed, but for most DIYers the expense is hard to justify. Used meggers are also available from auctions. If you only need it once, resale after use can cut costs.

Multifunction testers aren't really aimed at diy use.

Old hand cranked meggers occasionally turn up. They're not particuarly accurate, but good enough for locating faults. Missing insulation can be a safety issue on historic equipment, but if you do get bit you'll certainly stop cranking it!

This article shows how to make a basic megger at a cost of almost nothing. The megger is very simple, can be assembled in a few minutes, and works like an old hand cranked megger.


Components

Turntable motor from nuke 5671-3.jpg
  • Microwave oven turntable motor (from a scrap nuke)
  • Bridge rectifier 1kV rated (or 4x 1kV diodes, such as 1N4007)
  • 1.8uF or 2.2uF 1kV capacitor
  • 5x 1 Megohm resistors

Optional:

  • A case for the parts
  • Output connectors of any convenient well insulated type

How it works

This is a hand cranked dc high voltage generator, and is used with 2 moving coil multimeters to read the applied voltage and leakage current.


Meters

Moving coil meters are used because they handle unsteady readings well. Digital meters are unusable.

Construction

It can be built into a box as a permanent piece of test equipment, or just clipped together with croc clip leads for one-off use. If the latter is done, be sure to arrange the wiring so that a slip can't connect you to any high voltage bits. An easy way to do that is to put the turntable motor on its own lead away from the other bits, wrapping the lead round something secure.

       ___                                _
   ___|  +|-----+-----+----o     ---+----(i)----- L&N -----+
 _/   |~  |   + |     |             |                      |
(_)   |   |    ===    R            (V)                   [UUT]
  \___|~ _|     |     |             |                      |
      |___|-----+-----+----o     ---+-----------  E -------+

Gen    BR       C    5Rs            V     A           Unit Under Test
  • Gen: Turntable motor, hand cranked
  • BR: Bridge rectifier, or 4 diodes arranged as a BR
  • C: Capacitor
  • 5Rs: 5x 1Megohm resistors all in series
  • (V): Voltmeter
  • (i): ammeter
  • UUT: Unit under test, usually an appliance or section of fixed wiring


Operation

Connect the voltmeter across the generator's output. Connect current meter and UUT (unit under test) in series, and across the generator output.

Note the current flowing through the voltmeter does not also flow through the ammeter. (If the meters are connected in the wrong positions, the ammeter will read a current when none is flowing to the appliance.)

Turn the knob on the motor at the speed that gives the desired test voltage. 500v is enough for most jobs. Read the leakage current. 1kV isn't usually needed.

Don't crank the motor above 1kV. Exceeding the diodes' voltage ability, even momentarily, is likely to kill them. (Survivable voltage is often a bit higher than rated voltage.)

To test an appliance, connect one of the output wires to both L and N on the appliance, and make sure any power switch on the appliance is on. The other megger wire goes to the appliance case. For plastic cased goods, the case wire is touched to all the various bits of exterior metalwork, such as screws.


Safety

Never construct or use this machine unless you're competent to work on live electrics. Turntable motors can produce around 1,000v output.

Although output voltage is high, current output is very low, limiting the risks involved in high voltage work. However such machines are capable of inflicting serious injury in foolish or untrained hands, so should be secured away from children and grown ups that lack understanding of the risks present.

Fit a shorting wire when stored so accidental handle movement doesn't produce a dangerous voltage.

The risks in using meggers are significant, and can appear on electrical wiring a long distance from the user. Don't use a megger unless you're able to manage such risks safely.

More information: Wiki Safety


Accuracy

These meters aren't accurate, but they're plenty good enough for fault finding. The difficulty of maintaining a constant and precise crank speed means the output voltage varies quickly. They're more than accurate enough to detect insulation breakdown problems, enable checking of repairs, and test one's own appliances for insulation failure.

Its also posible, though not ideal, to use them to ensure wiring meets a minimum resistance level, by testing to far enough above the required resistance to be confident the insulation complies.

Accuracy can be improved a fair bit by adding a handle to the motor spindle, enabling continuous and more constant speed, and thus steadier output voltage.

These meggers don't meet current British Standards and aren't legal for use at workplaces, PAT testing, etc.


See Also

Personal tools