Circuit Energizes on Bypass but Not Through the Normal Switch: Decision Tree

Why this matters

When a load comes alive the instant you jumper across the switch, contactor, or control device, but stays dead through the normal control path, the fault is in the control path, not the load and not the supply. This is a clean, satisfying split because the bypass test has already proven the line side is hot and the load side is healthy. The work now is to find which element of the control string has gone open or high-resistance: the switch contacts, the coil and its control voltage, the pilot device, the interlock, or the conductors between them. Done in order, this is a fifteen-minute diagnosis. Done by guessing, it ends in a replaced switch that did not fix anything.

Symptom presentation

The technician lifts a jumper across the line and load terminals of a switch (or across the power poles of a contactor) and the load runs. Remove the jumper, operate the switch normally, and nothing happens. Variations: the switch clicks but the load does not energize (a contactor with a dead coil), the toggle moves with no continuity change (failed switch contacts), or the control works sometimes and drops out (a marginal contact or a sagging control voltage). Three-way and multi-location switching multiplies the suspects because any traveler, common, or companion switch can open the string.

Quick checks

• Confirm the bypass was across the right pair. Bypassing line-to-load on a manual switch proves the switch; bypassing the power poles of a contactor proves the power side, not the coil circuit. Know which you tested.
• For a contactor or relay, listen and look. A clicking armature with no load means open power contacts; no click at all means no coil voltage.
• Meter the control voltage at the coil or at the switch's incoming terminal. A 24 V control coil reading 0 V, or a line-voltage switch with no voltage at its common, points upstream in the control string.
• Operate the device and watch for continuity change across its contacts (power off and locked out for a manual switch, or with a meter rated for the live control test you intend to perform).

Isolation tree

Branch 1: Manual switch, no continuity when closed. The switch contacts have failed open, or a traveler/common is mislanded. On a single-pole switch, power-off continuity across the two terminals should close when the toggle is thrown. No change means a dead switch. On a three-way, verify the common terminal of each switch and chase the two travelers; a swapped common-and-traveler will make the circuit work from one position only or not at all.

Branch 2: Contactor or relay, coil dead. Meter the coil terminals while commanding ON. If coil voltage is present but the contactor does not pull in, the coil is open (ohm it cold, power off: an open or wildly out-of-spec coil is the fault) or mechanically stuck. If coil voltage is absent, the fault is upstream of the coil: the pilot device (thermostat, pressure switch, float, time clock), the control transformer, a blown control fuse, or an interlock contact.

Branch 3: Contactor pulls in but no power passes. The power contacts are pitted or welded open. Meter line-to-load across each pole with the contactor energized; a large voltage drop or no continuity across a closed pole confirms burned contacts. Replace the contactor; do not file contacts on a sealed device.

Branch 4: Interlock or safety opens the string. Many control circuits route through limits, overloads, door interlocks, or e-stops in series. The bypass jumped all of them. Meter each series element: a normally-closed safety that reads open under the present condition is doing its job or has failed. Confirm whether the condition is real (an actual overtemp, a tripped overload, an open door) before defeating it.

Branch 5: Control voltage low. A control transformer feeding a long run, or a shared control supply overloaded by added devices, sags below the coil's pickup voltage. Meter at the coil under the command; a 24 V coil that sees 16 to 18 V may chatter or fail to pull in. Find the drop and the loaded source.

Confirming diagnosis

You have it when energizing or de-energizing the one suspect element reproduces the symptom on demand.

• Switch contact failure: continuity stays open across the closed switch; replacing or rewiring the switch restores normal operation.
• Coil failure: coil reads open or out-of-spec cold, or sees correct voltage yet will not pull in.
• Power-contact failure: measurable drop or open across a closed pole under the energized contactor.
• Control-voltage starvation: coil voltage measured below the device pickup spec while the rest of the string is intact.

The proof is that the load now runs through the normal control path with the bypass removed.

Remediation

• Replace failed manual switches and three-way switches; re-verify common and traveler landings against the actual circuit.
• Replace contactors/relays with open coils or burned power contacts as a unit; match coil voltage and contact rating.
• Correct or replace the failed pilot or interlock device only after confirming the protected condition is normal.
• Restore control voltage: replace a failed/undersized control transformer, clear an overloaded shared control bus, or shorten/upsize the control conductors causing the drop.
• Re-land any swapped travelers and re-test from every switch location.

References

• NFPA 70 (National Electrical Code), Article 404, Switches; Article 430 Part VII, Motor Controllers and contactors.
• NFPA 70, Article 725, Class 1, 2, and 3 control circuits and conductor requirements.
• NFPA 70E, Standard for Electrical Safety in the Workplace, for energized control-circuit testing and PPE.
• NEMA ICS 2, Industrial Control and Systems: Controllers, Contactors, and Overload Relays, for contactor coil pickup and contact ratings.
• UL 508, Industrial Control Equipment, for control-device construction and ratings.