Bell transformer short: electrical burning smell in house

Troubleshooting by phone saves customer service call expense

I recently had a call from a concerned customer, about an electrical burning smell in the house, after demolishing their own kitchen in preparation for a renovation and remodel. Sweeping up afterward, they saw some slight sparking on old wires that had had a lighted doorbell button connected.

With a few more questions, I quickly steered her into the basement and toward the following theory:

• The bell wires had become crossed (shorted).
• Short-circuit current through the bell system transformer operated the transformer at high current for extended periods (whereas it is designed to operate through the resistance of the bell button lamp when at rest, or the bell motor/chime when the button is pressed — momentarily not continuously).
• The bell transformer windings (tiny copper insulated by a tiny layer of insulation to allow the windings to pack densely together for maximum effect / minimum size) burned through their insulators and/or opened the circuit by burning a winding conductor out like a lightbulb filament. This was the burning smell.

Subsequent inspection bore out the theory, and the customer has had the transformer disconnected from the circuit by a local handyman friend, rather than call for outside service.

Background:

Bell, intercom, and thermostat/control systems typically operate on low voltage AC circuits (under 48V), allowing the use of freer wiring methods, including smaller wires (16-22 guage), exposed wiring acceptable, and splices without junction boxes acceptable. Bare skin will not pass current (permit a shock) below 70V (women typically can’t be shocked below 85V and men below 90 or 100V), and building structure is similarly safer from low-voltage wiring than 120V household power wiring.

Transformers energize the bell/control systems using power from the household 120V power system. AC on the transformer input coil induces a ratio of AC voltage on the transformer output coil corresponding to the number of windings in the two coils. A typical winding ratio of 10/1 yields 12V AC from the 120V AC input.

Since the failure mode of transformers (usually due to a short circuit, or continuous rather than intermittent use) is to open the circuit, even short circuits on the low voltage side aren’t likely to present a fire or personnel safety issue. Typically, the transformer acts as its own circuit breaker.