Page 52 - Energize April 2022

TECHNICAL

Insulation resistance testing of

surge/lightning arresters

INFORMATION FROM MEGGER

lectric power transmission and distribution systems are vital once the applied voltage rises above normal (see Figure 2). They
for modern life. And, since the electricity these systems can do this by using discs of MOV (Metal Oxide Varistors), like zinc
Eprovide is essential, then these systems are essential too. oxide which has the property of changing resistance based on the
One of the less celebrated components of these systems is the voltage applied.
humble surge arrester. These arresters serve the important As can be seen in the graph of Figure 2, an arrester has a high
job of keeping the electric supply smooth and steady. As such, and constant (straight line) resistance in the operating range.
these devices must be tested regularly as part of a maintenance Above that, the resistance drops dramatically allowing a lot of
program. current to pass through it.
This application note looks at what a surge arrestor is, the test So, an arrester’s main task is to maintain the optimal voltage
procedures of insulation resistance testers, as well as present test condition of the other parts of the system by diverting or drawing
results and recommendations. excess electric power away. They are installed by connecting
one side to earth/ground and the other to the system but in
What is a surge arrester? parallel with the asset they are protecting. Usually, the arrester is
A surge arrester is a key component of electrical transmission protecting a transformer (see Figure 3).
and distribution systems. They are commonly found on top of
transformers and can change their resistance depending on the Please note that testing at these voltages can be very dangerous
voltage applied across them. The applied voltage can rise because unless you take the necessary precautions, these include but are
of lightning (see Figure 1) or system faults, among others. not limited to:
Arresters have been designed to have a high resistance at • Correct use of PPE e.g. electrically certified safety boots, safety
normal operating voltages, which can then drop to low resistance gloves etc.
• Making sure the equipment has been discharged and safely
connected to earth/ground
• Making sure that the test subject and the equipment are
isolated to make the test as safe as possible
• Test with a voltmeter before proceeding to make sure there are
no hazardous voltages present
• Visually inspect all equipment for damage/faults before any
testing

Insulation testing an arrester must be done based on its rated
voltage. So, always test with at least 50% of the rated voltage
no more than 120%. For example, a 10 kV rated arrestor can
be tested with at least 5 kV and no more than 12 kV. This is
Figure 1 necessary because this is the range where an arrester’s resistance
is constant while also being a high number.
Use the appropriate size test clips for the arrester under test
and attach the black lead to the top of the arrestor and the red
lead to the bottom. Finally, attach the blue lead to the middle of
the arrester using a conductive tape or wire (see Figure 4).
The three terminals play an important role in the
measurement. The test voltage is applied between the black and
red terminals, while the blue terminal is for measuring surface
leakage to discount it from the final result. The blue terminal
is the guard terminal, and it is vital for getting an accurate
Figure 2 measurement of resistance.

energize | April 2022 | 50