Due to the high sensitivity of ground-fault-current detection circuits, it is possible that excessive line noise on the power source wiring can cause an alarm. This can occur in the form of a ground-fault alarm, a stuck-relay alarm, or a ground-fault circuit alarm.

This type of alarm can be caused by switching high-power loads, inductive loads, or any excessive arcing during operation of a contactor that is on the same circuit branch. It may also be caused by extreme levels of RFI (radio-frequency interference) in the area.

Line Noise Issue Solutions

1. Use a separate circuit for the heat control

We recommend providing a separate circuit for the heat control, which is not shared with other equipment. In particular, any equipment that is electrically noisy needs to be on a different circuit branch and installed a safe distance away.

2. Use an external noise filter

The controller’s immunity to excessive conducted noise from contactors, inductive loads, and other sources of RF interference can be increased with an external noise filter. A simple way to provide additional filtering is to route the power supply cable through a ferrite toroid ring, with as many turns as possible. This will further attenuate the conducted RF noise.

One device that works well is the Palomar Engineering Ferrite Toroid Ring model F240-77. This is 1.4” ID and 2.4” OD, material type 77(Figure 1). There are larger sizes and other suppliers of these which also can work. We purchased our Ferrite Toroid Ring from here, but they can also be purchased at many electronic supply companies or on amazon.com.

This should be installed on the power line (source) cable to the controller (e.g. GPT or FPT), and placed near the controller’s housing (Figure 2). If the controller is fitted with conduit for the wiring to the power panel, the toroid can be placed just inside the power panel, at the end of the conduit (Figure 3). Three turns of the entire cable through the toroid are recommended; if more can be fit that will work better – the attenuation of noise is proportional to the square of the number of turns. A larger toroid can be used to accommodate more turns of the wire.

3. Adjust the ground fault threshold

In some situations, the ground fault threshold can be increased, and this will improve the noise immunity. On the GPT 130 and GPT 230, the ground-fault alarm current threshold can be adjusted from 1 mA to 300 mA, and the default setting is 30 mA. A higher setting will be more tolerant of electrical noise on the power line.

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