ground secondary of control transformer

[doug7777]

I am trying to understand why people ground the secondary of a low voltage (24 VAC) control transformer. I have seen this in control panels or rooftop units.

Is this a code requirement and why? I thought basically the only reason for grounding is safety from electrocution. If this is the only reason it makes sense to me. I would also think any devices powered by the control transformer should be grounded for protection from electrocution while working on the device.

I also hear that in control panels some people ground the DC circuits like the output of a 24 volt DC power supply. Why would you ground DC circuits?

 

[iwire]

Re: ground secondary of control transformer

In both low voltage AC and DC grounding one side makes troubleshooting easer.

 

[don_resqcapt19]

Re: ground secondary of control transformer

If the main power to the rooftop unit is 480, the code requires that the secondary of the 24 volt control transformer be grounded.
Don

 

[petersonra]

Re: ground secondary of control transformer

There are a couple reason to ground 24VAC or DC control power.

1. It makes debugging a lot easier, since you can use the steel frame of what ever you are working on as one side for testing voltages.

2. it can reduce some kinds of noise.

3. It reduces the number of OCPD devices in half.

4. Especially for DC control, you can force all your different DC commons to the same point, ground it and you can be sure none of them will ever exceed your maximum DC voltage to ground allowed by many comm circuits.