7 Commonly Asked Questions about Surge Arresters

1. What is the difference between a grading ring and a corona ring?

A grading ring is used to ensure a uniform voltage distribution along the length of an electrical device.  This is important for surge arresters so each MOV disc in the arrester is energized at the appropriate voltage.

A corona ring is traditionally used to electrically shield external hardware to prevent corona from developing.  This corona could lead to degradation of insulating materials or create interference to electronic communication. 

2. What does the MCOV rating of a surge arrester mean?

MCOV stands for the Maximum Continuous Operating Voltage.  It represents the power frequency voltage that may be continuously applied to a surge arrester. 

The MCOV selected for a given system voltage is a function of the maximum line-to-line voltage as well as the system grounding parameters. 

3. How does MCOV rating differ from Duty Cycle rating?

The Duty Cycle rating of a surge arrester is the power frequency voltage at which the arrester can successfully withstand the duty cycle test per IEEE Standard C62.11.  The Duty Cycle rating is a short-term TOV (Temporary Over Voltage) rating.

4. What is the difference between a station class and an intermediate arrester?

Generally, station class arresters have the lowest protective characteristics and most durability, while intermediate arresters perform at levels slightly less robust than station class arresters.  Both of these arresters have traditionally been used in sub-station applications. The arrester IEEE Standard C62.11 defines the performance levels of each of these designs. 

5. Why is the system grounding type important to consider when selecting the MCOV rating?

The type of grounding determines the amount of neutral shift during a fault on the power system. The resulting TOV on the arrester could cause damage unless the arrester is sized properly. 

6. How do I use the pressure relief rating value in making my arrester selection?

When a surge arrester fails, it will become shorted.  It then will conduct the available short circuit current in the substation.  To minimize the possibility of a catastrophic failure, you should select an arrester with a pressure relief rating that is greater than the available short-circuit current in your substation.

7. What routine maintenance and testing is done on station class surge arresters?

Arresters typically do not require field testing; however the following can be done as part of commissioning & maintenance tests :

• Visual and Mechanical Inspection.
• Verify sizing and rating for application.
• Verify proper anchorage, alignment, grounding, and clearances.
• Verify the  no surface contaminants are present.
• Inspect bolted electrical connections with a torque wrench.
• Inspect the  ground leads on each arrester is individually attached to a ground bus or ground electrode.
• If equipped with a stroke counter, ensure it is correctly mounted and electrically connected.
• Perform ground integrity test with a Digital Low Resistance Ohmmeter(DLRO).
• Perform a DC insulation-resistance test on each arrester, phase to ground. Applied voltage level must be well under the MCOV rating of the arrester.
• Perform a watts-loss test & current measurement( PF). Applied voltage level must be well under the MCOV rating of the arrester.
• Perform Infra-Red thermography.