Safety first: a capacitor can stay charged even when the bank reads zero
Do this before touching a single terminal:
- De-energize the bank and lock it out. Wait at least the discharge time printed on the nameplate.
- Measure the voltage at every capacitor — between terminals and from each terminal to ground. Do not trust the busbars reading zero: an open fuse, a stuck contactor or an open discharge resistor can leave one capacitor charged while the rest of the bank reads zero.
- Short and ground the terminals with a hot stick or an insulated discharge jumper. Qualified personnel only.
- A capacitor that has been discharged can build voltage back up on its own. Keep the jumper on, take it off only for the reading and put it back right after the measurement.
- Disconnect the capacitor from the bank before measuring, so the meter sees that unit alone.
The three steps
1. Read the nameplate
Find the capacitance per element, for example Cn = 3 × 96 µF, −5/+10 %: three elements of 96 µF each and a delivery tolerance of −5 % to +10 %. Also note the connection (delta or star), the rated voltage and the discharge time.
2. Work out what the meter should read
In a three-phase capacitor connected in delta, between any two terminals (the third one left free) the meter sees one element in parallel with the other two in series: 1.5 × C of one element. With 96 µF per element: 1.5 × 96 = 144 µF.
In star, between two terminals the meter sees two elements in series: C ÷ 2.
3. Measure and compare
With a capacitance meter, measure between L1–L2, L2–L3 and L3–L1. Each reading should fall inside the delivery range, −5 % to +10 % of 144 µF: 136.8 to 158.4 µF.
Check your capacitor
Enter the nameplate value and, if you have them, your three readings. Everything runs in your browser; nothing is sent anywhere.
Result
Enter the capacitance per element to see the result.
A guide, not a certificate. It assumes three equal elements, a capacitance meter in good condition and a capacitor disconnected from the bank. Follow your site’s electrical safety procedure and your capacitor manufacturer’s instructions.
Aging: how to read the result
A capacitor loses capacitance as it ages and as elements inside it fail. Compare each reading with the expected value:
| Reading vs. expected | Meaning | What to do |
|---|---|---|
| Above 110 % | Outside the delivery range | Check the measurement: meter, leads, range, and whether the capacitor is really isolated from the bank. |
| 95 % to 110 % | Within tolerance | Healthy. Record the value to compare it next time. |
| 90 % to 95 % | Low, but above 90 % | Still usable. Measure again sooner and watch the trend. |
| Below 90 % | Review | Review the unit: inspect it, repeat the measurement and plan its replacement. |
| Below 75 % | Replace | Replace it. |
The 90 % and 75 % limits are practical criteria, not a standard. Confirm them against your capacitor manufacturer’s criteria and your own maintenance procedure.
Watch the balance. The three readings of a healthy capacitor are close to each other. If one is clearly lower than the other two, an internal element may be damaged, even when the average still looks acceptable.
Extra: how many kvar does it deliver?
Reactive power depends on the square of the voltage and on the frequency. For a delta capacitor, with V between phases and C per element:
In star: Q = V² · 2π · f · C.
Example: the same 3 × 96 µF at 60 Hz delivers 29.9 kvar at 525 V and 25.0 kvar at 480 V. A capacitor connected to a lower voltage than the one its nameplate kvar refers to delivers fewer kvar, and one that has lost capacitance delivers proportionally fewer.
Keep the history of every bank
A reading is only useful when you can compare it with the last one. With Wertek AI you can:
- Register each capacitor bank as an asset, with its nameplate data.
- A maintenance plan that creates the work orders on their due date.
- Technicians receive the orders in the mobile app on Google Play, record the work and close them.
Register your capacitor banks
Registering the bank and its nameplate in Wertek AI is free. Work orders come with the paid plans.