The Case of the Non-Tripping Safety Circuits


ACES performs the annual gas train safety checks on the ovens and autoclaves for an aerospace parts provider.

One of steps of the safety checks is the combustion blower airflow or pressure-proving switch. When the ACES Control System Investigator was performing the blower/pressure-proving switch check on a large, fairly new oven, he discovered it would not trip.


The CSI did a pressure test on the switch and found the pressure line was incorrectly plumbed to the vacuum side of the switch. After that he investigated the switch to verify it was wired to the normally open contact set (per NFPA 86), which closes on pressure rise.

The switch was wired correctly so the CSI put a voltmeter on the terminals. He found was there was 120 volts present on both terminals when voltage should have been present on only one terminal. Only after the switch is made should there be voltage at the second terminal. Where was the excess voltage coming from?


When he examined the main control cabinet The CSI discovered the problem — there was a jumper wire on the terminal block applying voltage to both sides of the pressure switch.

The jumper wire was tricking the safety circuit into believing the combustion blower was functioning properly. If there had been a problem with the blower the safety circuit would have been unable to sense it. The whole oven chamber could have filled with raw gas, and if the operator neglected to perform a complete purge when he relit the burner, there could have been an explosion.


The first thing the CSI did was move the pressure line from the vacuum port to the pressure port of the switch. With the switch properly plumbed he was able to remove the jumper wire.

Now the client is in compliance with safety standards and can operate smoothly, without risk of damage to people or property

Perform your annual gas train safety checks and never bypass any safety circuits.

ACES is often called to the scene of improperly-installed new equipment. If your new machinery isn’t performing as expected, call the control system experts at ACES.