Ohmmeter and Megohmmeter

Ohmmeter and Megohmeter

Ohmmeter
An ohmmeter is an electrical instrument that measures electrical resistance -- the opposition to an electric current.

Micro-ohmmeters (or microhmmeter) make low resistance measurements. Megohmmeters (or in the case of a trademarked device Megger) measure large values of resistance.

The unit of measurement for resistance is ohms (Ω).

Ohmmeter – Original Design
The original design of an ohmmeter provided the following:
• A small battery to apply a voltage to a resistance.

• A galvanometer to measure the electric current through the resistance.

The scale of the galvanometer was marked in ohms, because the fixed voltage from the battery assured that as resistance decreased, the current through the meter would increase.

Note:

Ohmmeters form circuits by themselves; therefore, they cannot be used within an assembled circuit.

A more accurate type of ohmmeter has an electronic circuit that passes a constant current (I) through the resistance, and another circuit that measures the voltage (V) across the resistance.

Megohmmeter
A megohmmeter is a special type of ohmmeter used to measure, or rather test, the insulation resistance of insulators.

Insulators have electrical resistance in the range of millions of ohms compared to conductors which are measured in ohms.

Insulators need to be tested for their insulation strength at the time of commissioning. Testing is also a part of the maintenance process for electrical equipment and installations.

For this purpose, special ohmmeters called megohmmeters which can provide high voltages (typically in ranges from 500V to 2 kV) at specified current capacity are used.

Note:
The minimum insulation tolerated is in the range of 1 to 10 mega-ohms, depending on the standards referred to.