Difference between revisions of "Megger"

Revision as of 23:07, 10 April 2010

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

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

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 aren't unsuitable.

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.

@@ Line 1: / Line 1: @@
-A '''Megger''' can be bought when needed, but for the average DIYer the expense is hard to justify. This article shows how to make a basic megger at a cost of almost nothing.
+Meggers test the insulation of appliances & wiring with 500-1000v.
+==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.
-This megger is very simple, and works similarly to old hand cranked meggers.
 ==Components==
-* Microwave oven turntable motor from a scrap machine
+* Microwave oven turntable motor (from a scrap nuke)
-* Bridge rectifier 1kV rated (or 4 diodes)
+* Bridge rectifier 1kV rated (or 4x 1kV diodes, such as 1N4007)
-* 2.2uF 1kV capacitor
+* 1.8uF or 2.2uF 1kV capacitor
-* 5x 1Megohm leakage resistors
+* 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 micropower dc high voltage generator, and is used with 2 moving coil [[multimeter]]s to read voltage and current.
+This is a hand cranked dc high voltage generator, and is used with 2 moving coil [[multimeter]]s to read the applied voltage and leakage current.
 ==Meters==
-Moving coil meters are used because they handle unsteady readings well. Digital meters aren't recommended as they don't behave well with unsteady readings.
+[[Multimeter|Moving coil meters]] are used because they handle unsteady readings well. [[Multimeter|Digital meters]] aren't unsuitable.
+==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 in series, and across the generator 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.)
-    ___|  +|-----+----(_)----o   L&N -----+
-  _/   |~  |     |                        |
- (_)   |   |    (_)               Unit Under Test
-   \___|~ _|     |                        |
-       |___|-----+-----------o   E -------+
- Gen    BR       V     A
-Note how the current flowing through the voltmeter does not also flow through the ammeter. If connected wrong, the ammeter will read current wrongly.
+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.
-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.
+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.)
-Do not crank the motor to above 1kV. Doing so, even briefly, is liable to destroy the various components.
+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 are competent to work on live electrics. Turntable motors can produce around 1,000v output.
+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.
-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 damage in foolish hands, so should be secured away from children and grown ups that lack understanding of the risks present.
+The risks in using meggers are significant, and can appear on electrical [[cable|wiring]] a long distance from the user. Don't use a megger unless you're able to manage such risks safely.
-Fit a shorting strip when stored to prevent any external voltage resulting from accidental handle movement.
+More information: [[Wiki Safety]]
 ==Accuracy==
-These meters can not be considered accurate. The difficulty of maintaining a constant and precise crank speed means that output voltage varies fairly quickly in practice. However they're plenty accurate enough to detect insulation breakdown problems, enable checking of repairs, and test one's own appliances for insulation failure.
+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 significantly by adding a crank handle to the motor, enbling continuous and more constant speed and thus output voltage.
+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 BS and aren't suitable for use at workplaces.
+These meggers don't meet current British Standards and aren't legal for use at workplaces, PAT testing, etc.
 ==See Also==
+* [[Wiki Safety]]
 * [[Special:Allpages|Wiki Contents]]
 * [[Special:Categories|Wiki Subject Categories]]