How to Troubleshoot an Antiquated Knife Switch

Knife switches were commonly used to control and isolate electrical circuits in the early 20th century before safer enclosed switches became standard. While antiquated, knife switches can still be found in older buildings and industrial settings. Troubleshooting these outdated switches requires care and a basic understanding of their simple yet sometimes dangerous mechanisms.

Assess the Overall Condition

Before attempting to diagnose specific issues, take time to thoroughly examine the overall switch. Check for signs of:

Carefully inspecting the overall condition gives you an idea of the extent of any repairs needed before attempting to restore operation.

Check for Proper Switch Action

The actual switching operation of a knife switch is very simple - the blade separates from or engages with the jaws to open or close the circuit.

With the switch presumably off, manually move the switchblade back and forth between the open and closed positions. Note any problems like:

Testing the mechanical operation gives clues about what maintenance or repair tasks are required.

Check Circuit Continuity

Use a multimeter to methodically check circuit continuity through the switch contacts.

First, verify continuity from one fixed jaw to the other with the switchblade removed. This checks the jaw contacts independent of the moving blade.

Next, reinstall the blade and check continuity in both the open and closed positions. The closed position should show continuity while the open position breaks continuity.

Any deviation points to contamination or a weak connection at a specific spot in the current path. Target cleaning and mechanical adjustment at those spots.

Checking continuity pinpoints where an electrical problem may exist. Correlate discontinuities with any mechanical issues noticed during motion tests.

Thorough Cleaning

A thorough cleaning is the first step in revitalizing an antiquated knife switch.

Proper cleaning is essential for both electrical conductivity and smooth mechanical operation. Always de-energize and disconnect the wires before cleaning.

Adjusting and Lubricating

Proper mechanical adjustment and lubrication helps ensure smooth snap-action performance.

Getting the right mechanical adjustments is key to achieving good performance and minimal arcing.

Replace Damaged Components

For badly damaged switches, replacement of irreparable components may be necessary:

Component replacement restores performance and safety. Consider a professional electrician for major repairs.


While antiquated, knife switches are repairable and serviceable with careful attention. Thorough inspection, cleaning, adjustment, lubrication, and replacement of damaged parts can often restore proper and safe operation. However, upgrades to modern enclosed safety switches are recommended where feasible.