The Ins and Outs of Replacing Knife Switches in Older Commercial Buildings

I am often tasked with replacing old knife switches in the commercial buildings I service. While daunting at first, I've learned a few key things that make the process much easier. Here's an overview of the ins and outs of replacing these outdated switches.

Understanding Knife Switches

Knife switches, sometimes called disconnect switches, are typically found in older commercial buildings. They were commonly installed in the early to mid 1900s.

Knife switches are bulky electro-mechanical switches that manually connect and disconnect electrical circuits. They consist of a hinge design with exposed metal conductors that are "opened" or "closed" to break or complete the circuit.

The switch gets its name from the knife-like blades that are moved to open or close the circuit. The large levers allow quick and easy on/off control of equipment and circuits.

Knife switches provide overcurrent protection and a means to manually isolate parts of an electrical system. This allows maintenance and repairs to be done safely.

However, they contain no automatic safety mechanisms. Newer switches have safety features like fuses or circuit breakers to prevent issues like arching and overloads.

When to Replace Knife Switches

There are several signs that indicate it's time to replace knife switches:

Age: Switches over 50 years old likely need replacement. They become unreliable with long-term use.
Frequent failures: If you're constantly resetting or replacing failed switches, new ones are needed.
Lack of overcurrent protection: No fuses or other safety mechanisms means the risk of issues is higher.
Inability to fully isolate circuits: If switches don't fully open the circuit for lock out/tag out procedures, they pose safety risks.
Insufficient amperage ratings: Upgrades to electrical systems may require higher amperage switches.

Replacing older knife switches reduces operation and maintenance costs while also improving safety. I recommend replacement as part of any electrical upgrade project.

Choosing New Disconnect Switches

Modern disconnect switches are safer and more reliable than old knife switches. Look for these key features when choosing a replacement:

Higher amperage rating than old switch
Quick-make, quick-break operation for safety
Overcurrent fuses or circuit breakers
Padlockable handles for lock out/tag out compliance
Non-teasible blades that fully isolate and don't allow arcing
Durable sealed housing for indoor/outdoor use
Easy side wiring access for installation

Consult electrical codes to determine optimal voltage and amperage ratings. I also take future expansion needs into account.

Installing New Disconnect Switches

Installing a new disconnect switch involves working safely with live electrical systems. I take the following steps:

Turn off power and verify it's off at the circuit breaker/fuse box. Use a multimeter to double check there is no power.
Disconnect and remove the old switch. Carefully detach all power line connections. Unbolt the switch from the wall and dispose of properly.
Install mounting brackets and bolt the new switch securely in place based on manufacturer instructions.
Attach power line connections to the new switch terminals. Check them carefully for tightness.
Re-energize the system and test operation of the switch multiple times to verify smooth on/off control.
Apply warning labels as needed to indicate live voltage. Update any wiring diagrams/schematics.

Safety comes first! I use proper PPE and follow lock out/tag out procedures. Taking the time to install new switches correctly avoids issues down the road.

Maintaining and Testing Disconnect Switches

Once installed, disconnect switches need minimal maintenance. I follow these tips to keep them operating properly:

Annually test operation and visually inspect terminals, blades, and housings for any damage.
Check fuses/breakers to ensure proper sizing. Replace as necessary.
Keep switches free of dirt and dust buildup which can impede performance.
Lubricate moving parts approximately every 5 years to prevent sticking.
Verify switch blades fully open circuits by using a multimeter. Adjust or replace the switch if conduction is detected.
Follow manufacturer maintenance recommendations.

Proper maintenance extends the life of disconnect switches. Periodic testing gives me confidence they will operate reliably when needed.

Conclusion

Replacing antique knife switches requires careful planning and adherence to safety protocols. The effort is well worth it to prevent electrical issues and improve reliability.

Choosing suitable modern disconnect switches, proper installation, and ongoing maintenance provides superior circuit protection. Workers can then isolate equipment with confidence for safe maintenance and repairs.

In summary, I've found updating old knife switches in commercial buildings, while at times challenging, pays dividends in safety and performance. Let me know if you need help assessing your old electrical switches!

“The Ins and Outs of Replacing Knife Switches in Older Commercial Buildings”