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Feeder switching guideline; contactor in place of CB

Short answer

To disconnect a feeder: open CB first, then line and bus isolators, then earth it. To connect: remove earths, close isolators, then CB last. A contactor cannot replace the CB because it cannot interrupt fault currents and lacks relay tripping; it is only for frequent load switching with fuse backup.

A [high?] voltage feeder has a circuit breaker and isolator. Develop a guideline for connecting and disconnecting the feeder through isolator and circuit breaker. Can you replace circuit breaker with contactor in the feeder? Justify your answer.

Taking the question as a high-voltage feeder (e.g. 33 kV) fed from a bus through isolators and a circuit breaker.

 Bus ==+==
       |
     [I1]  bus isolator
       |
     [CB]
       |
     [I2]  line isolator --[ES] earth switch
       |
     Feeder

Guideline for disconnecting the feeder

  1. Inform the control room and load end; reduce load if possible.
  2. Open the circuit breaker by the control switch; confirm "OFF" on the indicator and ammeter reading zero.
  3. Open the line isolator I2, then the bus isolator I1 (no-load operation).
  4. Check visibly that all three blades are open.
  5. Test for absence of voltage, then close the earth switch ES (and apply portable earths at the work site).
  6. Lock the isolators and CB, and attach "Do not operate – men at work" tags (permit-to-work).

Guideline for connecting the feeder

  1. Cancel the work permit; make sure all persons and portable earths are removed.
  2. Open the earth switch ES.
  3. Check that the CB is open; close the bus isolator I1, then the line isolator I2.
  4. Close the circuit breaker last; watch the ammeter and relay flags. If a fault exists, the CB trips safely.
  5. Record the operation in the log book.

Rule to remember: CB opens first and closes last; isolators move only when the CB is open. Interlocking enforces this.

Can a contactor replace the circuit breaker?

No, not on an HV feeder. Reasons:

  • A contactor is built to make and break normal load currents (and motor starting currents), with breaking capacity of only about 8–10 times rated current. A feeder short circuit can be tens of kA; the contactor would fail to interrupt it and could weld or explode.
  • A contactor has no tripping by protective relays in the usual sense; it opens only when the coil is de-energised, and it also drops out on voltage dips, causing unwanted outages.
  • Contactors are mostly made for LV and up to about 11 kV (vacuum contactors); they do not have the voltage rating, insulation level or making capacity of a 33 kV CB.
  • A contactor needs back-up fuses for short circuits; HV fuses large enough for a feeder are not practical and must be replaced after each fault.

A contactor is suitable only for frequent switching of a motor or capacitor bank; the feeder needs a circuit breaker with relays to clear faults.

✓ Answer checked by Claude Opus 5.5 (high effort)

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