Surge Arresters: Selecting MCOV by Grounding Mode

10 min · difficulty 5/10

A 12.47 kV grounded-wye feeder (7.2 kV line-to-ground) needs arresters at a riser pole. The catalog lists 9 kV and 10 kV duty-cycle (MCOV 7.65 kV and 8.4 kV) arresters. Which one? Choose before reading.

Pick the 9 kV duty-cycle arrester, MCOV 7.65 kV. A metal-oxide (MOV) arrester clamps a surge, but the rest of the time it sits across the line for years, so it has to survive the continuous voltage at its terminals without overheating. That continuous voltage is the line-to-ground voltage: 12,470 / 1.732 = 7.2 kV. The MCOV (Maximum Continuous Operating Voltage) must stay just above that, and 7.65 kV does. The 10 kV unit would also survive, but it clamps surges at a higher voltage, so it lets more through to the equipment you are trying to protect.

Grounding affects line-to-ground voltage and sets the surge arrester MCOV Two phasor diagrams on the left. In the grounded-wye normal case, three phase phasors sit 120 degrees apart around a neutral, each equal to the line-to-ground voltage of 7.2 kV. In the ground-fault case on an ungrounded or impedance-grounded system, the faulted phase collapses toward ground and the two unfaulted phasors lengthen by a factor of 1.732 toward the line-to-line magnitude. On the right, an MCOV selection ladder maps arrester duty-cycle ratings to maximum continuous operating voltage: 9 kV to 7.65, 10 kV to 8.4, 12 kV to 10.2, 15 kV to 12.7, and 18 kV to 15.3. The 9 kV to 7.65 row is highlighted because a 12.47 kV grounded-wye feeder runs a continuous line-to-ground voltage of 7.2 kV, requiring MCOV of at least 7.65 kV. MCOV follows the continuous line-to-ground voltage grounding sets V(L-G); the arrester must ride above it continuously (A) Grounded-wye, normal A B C 120° N (gnd) V(L-G) = 7.2 kV (B) Ground fault, ungrounded A B C faulted A → ground unfaulted phases rise toward V(L-L) 1.732 × V(L-G) MCOV selection ladder duty-cycle rating MCOV 9 kV → 7.65 kV 10 kV → 8.4 kV 12 kV → 10.2 kV 15 kV → 12.7 kV 18 kV → 15.3 kV 12.47 kV grounded-wye: V(L-G) = 12470 / 1.732 = 7.2 kV pick 9 kV duty-cycle → MCOV ≥ 7.65
Grounding sets the continuous line-to-ground voltage an arrester must ride above, so a 12.47 kV grounded-wye feeder at 7.2 kV V(L-G) needs at least 7.65 kV MCOV (a 9 kV duty-cycle arrester).

The rule is short. MCOV must be at least the continuous line-to-ground voltage the arrester sees. On an effectively grounded system that is line-to-line divided by 1.732, so you size MCOV just above it and stop. Bigger is not better here. A higher rating means a higher clamping voltage, which is weaker protection.

Grounding mode is the part people miss. On an ungrounded or impedance-grounded system there is no firm ground reference, so a single line-to-ground fault shifts the neutral and drives the two unfaulted phases up toward full line-to-line voltage. An arrester on a healthy phase has to ride that out continuously until the fault clears. That is why you step up to a higher rating at the same line-to-line class: the continuous duty is harder.

Size arrester MCOV from the continuous line-to-ground voltage, then raise it if the system is not effectively grounded, because a ground fault pushes the healthy phases toward line-to-line voltage.

Keep the standard pairs handy. Duty-cycle to MCOV: 9 to 7.65, 10 to 8.4, 12 to 10.2, 15 to 12.7, 18 to 15.3 kV. On a 24.94 kV grounded-wye feeder the line-to-ground voltage is 24,940 / 1.732 = 14,400 V, so the 18 kV duty-cycle unit (15.3 kV MCOV) is the grounded-system choice. Read the catalog as two numbers, duty-cycle and MCOV, and match the MCOV to the voltage your phase actually carries.

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Question 1 of 3

On a 12.47 kV grounded-wye feeder the line-to-ground voltage is 7.2 kV. The catalog offers a 9 kV duty-cycle arrester (MCOV 7.65 kV) and a 10 kV duty-cycle arrester (MCOV 8.4 kV). Which do you pick for a standard riser-pole application?

Educational material only. This is not engineering, safety, or procurement advice. Confirm any value against manufacturer documentation and a licensed professional before specifying equipment.