Megger MIT510, User Manual

across the insulation between the bare cable and the external sheathing

due to the presence of moisture or dirt. Where it is required to remove

the effect of this leakage, particularly at high testing voltages, a bare wire

may be bound tightly around the insulation and connected via the third

test lead to the guard terminal ‘G’.

The guard terminal is at the same potential as the negative terminal. Since

the leakage resistance is effectively in parallel with the resistance to be

measured, the use of the guard causes the current flowing through surface

leakage to be diverted from the measuring circuit. The instrument

therefore reads the leakage of the insulator, ignoring leakage across its

surface.

Factory programming socket:

This is a 9 pin socket under the hinged cover and is not for customer use.

B

button:

Pressing this button toggles the display backlight on and off.

"HV On" LED:

This is a red LED next to the 

TEST

button on the front panel. As long as

there is a voltage greater than 50 V, from an internal or external source, on

the test inputs, the LED flashes. 

"Charging Indicator" LED:

This is a green LED next to the 

Power On/Off

button on the front panel.

When mains power is present it shows green.

Test Terminals:

There are three test terminal , - and G. These terminals are

designed to accept only the test leads supplied. Shutters across the

terminals prevent accidental ingress of dirt and other objects. Test lead

plugs interlock with the shutters and are released by rotating the test lead

plug a quarter turn.

The Guard terminal is explained below and only used in cases where

surface leakage currents need to be eliminated. Most measurements use

just the + and – terminals. The instrument’s internal voltage generator

drives the + terminal with respect to the – terminal, current being

measured in the – terminal. 

The Guard Terminal:

For basic insulation tests and where there is little possibility of surface

leakage affecting the measurement, it is unnecessary to use the guard

terminal i.e. if the insulator is clean and there are unlikely to be any adverse

current paths. However in cable testing, there may be surface leakage paths

7