Rs485 & Utp

Beren

We have a contractor installing a generator for us. Everything is hooked up, but the generator does not automatically start at loss of utility. Our contractor so far hasn't been able to fix.

The automatic transfer switch is connected to the generator control panel via a RS-485 cable. The transfer switch is supposed to communicate voltage readings to the generator control panel, and the control panel decides whether to fire up the generator.

However, the control panel is not seeing any data from the transfer switch. To me, it sounds like either the port settings don't match, or the comm cable is not wired correctly.

Is it common to use CAT5 UTP for a two-wire RS-485 cable? They're using one pair twisted together for +, one pair twisted together for -, and one pair twisted together for shield. My understanding is that same pair should be used for + and -, with one wire going to + and one to -. Also, the cable should be shielded, not unshielded. Finally, the communications cable is run through the same conduit as the load lines instead of in a separate conduit.

fridaymean

Yup, sounds like you got some issues.

GregS

Yup. Using UTP is probably the problem and it should NOT be running in a conduit with your load lines.

It's also supposed to be a balanced signal, so the +/- 'should' be twisted in order to help with interference.

Get them to repull it with a proper wire.

ChillaWatt

In some cases, like remote control/signalling circuits, high and low can be ran in the same conduit. The low has to be associated with the power circuits, which seems to be your case. Should be shielded though.

Chilla

Beren

Thanks for the feedback. It turns out some things changed while I was on vacation. The contractors got it to start automatically...but it would always start as soon as it was set to auto. Another contractor found that the generator control panel had not been correctly configured.

While fixing the first problem, the original contractors either incorrectly set the auto-start voltage or didn't change the default. It was set to auto-start if the utility voltage dropped below 480V, which is bad when your normal voltage is 208.

The control cables work, and while the manuals state repeatedly that "control lines must be shielded and in a conduit separate from the load lines", no one seems to care so long as the generator starts.

Funny part was that they had wired the battery charger circuit (for the gen starter battery) into a light fixture. Guess what didn't charge when that light was turned off? That was fixed.