# Guessing power line voltage from the number of disc insulator

Is there a good way to guess the power line voltage based on the number of insulator discs that attach the line to the transmission tower?

Wikipedia seems to suggest that there is a "Typical number of disc insulator units for standard line voltages".

Is this a good ball park upper bound to the voltage on the power line?
Is there a better way of guessing line voltages?
Is it possible to further refine this guess based on the type of insulator discs?

• The total length of the insulator gives you some idea of maximum voltage it could withstand, since the limit is often the arc distance thru air, not what the material is made from. In any case, the arc distance is still there, so the voltage can't exceed that minus some derating factor. Jun 4, 2013 at 15:56

Is there a good way to guess the power line voltage based on the number of insulator discs that attach the line to the transmission tower?

We usually use as a rule of thumb for determining the number of porcelain/glass suspension disc insulators: 10kV for each disc (standard 5-1/4' x 10").

For typical system voltages in North America, this would be:
69kV: 4-6 discs; 115kV: 7-9 discs; 138kV: 8-10 discs; 230kV: 12 discs; 345kV: 18 discs; 500kV: 24 discs.

You may check Electric Power Generation, Transmission & Distribution by Grigsby, et al.

In general, probably not.

25KV Low Voltage Silicone Rubber Composite Tension Suspension Electric Insulator for Railway

Low Voltage 220kV Polymer Composite Power Line Insulator

In a specific country, with a single supplier of high-voltage transmission tower systems, for a small group of towers built at similar times, the answer may be yes.

• Your right this is a very location specific question. Most of california seems to have the same sort of high voltage distribution lines. aka the large metal transmission lines with what seem to be ceramic disc insulators, it seems like there might be a standard for modern ceramic disc insultors Jun 4, 2013 at 15:19
• The 220kV one is much larger (2240mm vs 480mm). Jun 4, 2013 at 19:07

See attached Table 4-6 from the USDA-RUS Bulletin 1724e300.

Usually, it is possible to say what voltage the insulator can withstand, because it is a function of its length. Typically these discs are set with the same distance so it CAN be possible but only as some approximation. The purpose of these discs is not however being markings of voltage, but (among others) they function to halt arc on the insulator surface.

The insulator length provides more information, but this is only design data. The line which was designed for 400 kV can be used as 110 kV, so it says nothing. The same information might be taken from distance between phase conductors, tower height (the distance between conductor and earth), distance from line to its neighbourhood and so on.

Taking a look on the line will not even show you if it is operational.

According to My research I have found that yes and no that you can and can’t base the line voltages off of the insulator number of discs.

Here is why you could NOT:

High voltages act differently in cold, hot, moist, or and dry areas. In Utah there are power lines with 12 ceramic discs on the power poles holding or suspending the power lines. The power lines only have 138 kV flowing through the power lines. But the number of insulating ceramic discs suggests that a little more than 161 kV is flowing through the power lines. So the answer for this is no, you couldn’t base voltages flowing through power lines on the number of ceramic discs on the power lines.

The reason why you could:

The main and only reason why you could base the voltage flowing through a power line off of the number of insulating discs is if you wanted to be over safe. Or what I call over-protective. Which means basically you are estimating higher than needed to make things even safer.