Originally posted by RKeener:
Regarding off-line testers. I own both Baker and All-Test and have used PdMA. All of these products have good Meggers.
The All-Test Pro testers that I have used did not have reliable meggers, in my opinion:
1. They did not display actual test voltage.
If, for example, you are conducting a 1000V test, and your test voltage drops to 800V, then the test is invalid, unless the resistance shows that the winding is shorted to ground. Most meggers display actual test voltage; the All-Test Pro meggers did not. They only displayed the voltage setting selected by the user.
2. I have watched All-Test Pro meggers pass windings that quality meggers failed. I also noticed that the displayed resistance values climb much faster with the All-Test Pro than with other meggers.
Out of curiosity, I measured current generated by four or five meggers with the leads shorted together. All others - even cheap meggers, costing less than $300 - produced around 1.6 milliamps. However, every All-Test Pro tester that I measured produced around 0.6 milliamp. I believe IEC 61557 specifies a rated current of 1 milliamp here.
3. The AT33 series motor tester differs from previous All-Test Pro devices in that the device can be used while it is being charged. However, the insulation resistance reading changed dramatically when the device was plugged in and charging.
4. The megger function locked up regularly (the screen froze, and the controls would not respond) on all All-Test Pro devices that I tested. Fortunately, the devices came with reset buttons, which were handy, but let's face it: most meters do not have this problem to begin with, and do not require reset buttons.