Capacitor bank discharge resistor — why a switched-off power capacitor can remain dangerously charged

The guide on power-factor correction covers why and how capacitor banks are applied to improve the power factor (cos φ) of an installation. This article covers an aspect that is not obvious in that context: what happens after a capacitor bank is disconnected from the mains, and why this requires a specific, standardised safety provision.

Why a capacitor stays charged after switch-off

A capacitor stores electrical energy in the form of an electric field between its plates — unlike a resistive load, which converts energy directly into heat and retains no residual charge once the current stops. If a capacitor bank is disconnected from the mains at a moment when the AC voltage does not happen to pass through zero, the voltage present at that moment remains across the capacitor as a DC voltage: the capacitor no longer has a path to discharge through unless one is explicitly provided. Without such a provision, an apparently switched-off capacitor bank can hold a voltage close to the peak value of the operating voltage for several minutes or much longer — for a low-voltage bank that can quickly mean several hundred volts.

The discharge resistor: a fixed, permanently connected provision

To remove this risk, every capacitor (or capacitor bank) is fitted as standard with a permanently connected discharge resistor, wired in parallel across the capacitor. This resistor forms a continuously present, but negligibly small loss path during operation, which gradually removes the stored charge after the supply voltage is disconnected. Because the resistor is permanently connected (unlike, for example, a switchable shorting link), no separate action is needed to start the discharge — it begins automatically as soon as the supply voltage disappears.

The standardised requirement: 75 V within 3 minutes

IEC 60831-1 (self-healing low-voltage power capacitors up to and including 1000 V) requires the discharge provision to bring the residual voltage across the capacitor below 75 V within 3 minutes of disconnection from the supply. This limit is chosen so that someone who needs to work on the terminals of the capacitor bank shortly after switch-off — for example for a replacement or inspection — is not exposed to a voltage that could still cause a dangerous shock, without requiring a separate, active discharging action every time.

Note: a capacitor bank with an integrated switch (for example an automatic controller that switches steps in and out) may, in addition to the fixed discharge resistor, also have a faster, electronically controlled discharge provision to allow the same step to be repeatedly switched in and out within seconds; the 75 V / 3-minute requirement of IEC 60831 nonetheless remains the baseline requirement that every capacitor (step) must meet at minimum, even without such a faster provision.

Why this is not the same as VFD DC-bus residual voltage

The guide on DC-bus residual voltage in variable frequency drives covers a related, but separate phenomenon: the residual voltage on the DC-bus capacitors of a variable frequency drive. Both cases revolve around the same underlying physics — a capacitor retaining charge after the supply voltage disappears — but apply to different equipment, with their own standards, their own discharge times and their own nameplate data; one discharge time must not be assumed to apply to the other application.

Practical relevance

Before working on a power-factor-correction capacitor bank — even if it appears switched off — the discharge time stated on the nameplate or in the documentation must always be respected (typically in line with the 3 minutes from IEC 60831), and the residual voltage must be explicitly checked with a suitable voltage tester before touching any terminal — a fixed discharge resistor is a standard requirement, not a guarantee against an individually faulty or interrupted resistor.

Common mistakes

Treating a capacitor bank as voltage-free immediately after switch-off — without the prescribed waiting time, the residual voltage can still be close to the peak value of the operating voltage.
Assuming the discharge resistor always functions without measuring the residual voltage — an individually faulty or interrupted discharge resistor is not visible from the outside and then produces no discharge at all.
Confusing the discharge time of a capacitor bank with that of a VFD DC bus or another capacitor application — the standardised discharge time and threshold voltage differ per equipment type and standard.
Short-circuiting a capacitor bank with an improvised piece of tooling "to be sure" instead of respecting the prescribed waiting time and measuring the residual voltage — this can itself cause an arc-flash and short-circuit hazard on a capacitor that is still significantly charged.