When oil switches are used as a building disconnecting means, where should an isolating switch be installed?

• A. On the load side
• B. On the supply side
• C. On either the load side or the supply side
• D. Not required

Answer: (C)

When an oil switch is used as the building disconnecting means, you still need a clearly isolating point that can be locked out for maintenance or emergency shutdown. That isolating switch can be placed either before the oil switch (supply side) or after it (load side). Placing it on either side provides a definite, observable disconnect and a safe way to de-energize the circuit for servicing, regardless of how the wiring is arranged. If it’s on the supply side, you’re isolating the source; if it’s on the load side, you’re isolating the building circuits. The important part is having a readily accessible, lockable isolating switch, which satisfies the safety requirement.