DC Power Supplies: To Ground or Not To Gound?

DC power supplies are a common, perhaps almost guaranteed, component of any control system. An often-discussed matter of DC power systems is the connection of the output to the ground system interconnecting industrial machines. Is that ground connection required? If not, is it a good or a bad idea to establish this connection?

Most DC power supplies installed within control cabinets output the common 24 volts. Computer power supplies, including PLC power supply units (PSU) usually output 5 V and +/- 12 V, all at a constant DC polarity. When examining the output wires, they only contain a + and a - terminal and rarely contain a protective earth (ground) connection. Yet for AC systems, this bonded ground system is regulated and required strictly.

Figure 1. Grounding power supplies inside a control cabinet can be a difficult decision. Image used courtesy of Canva

The matter of grounding DC supplies can be split into three areas: 

1. Examining the most critical aspect: Is it required by an NEC regulation? 
2. If not, it must be examined on a case-by-case basis, weighing the rationale for bonding the DC output to ground.
3. Finally, contrasting that with negative impacts that arise from such a connection.

Code Requirements for Grounding DC Power Supplies

The first essential question that must always be asked is: Is a connection required, and what conditions regulate that connection?

The answer comes from NEC Section 250.162, which refers to the grounding of two-wire DC systems, which includes the 5 V and 24 V outputs, depending on your case. The regulation sets a strict limit on the required grounding if the voltage is in excess of 60 V. Since neither of these two cases exceeds that voltage, we are then forced to weigh the possibilities and determine whether to make that optional connection.

So, the short answer for some 24 V DC systems is no, the output is not required to be connected to ground. From the UL 508A specification, there are further answers that also dictate grounding depending on the input voltage of the power supply.

Benefits of Grounding a DC Power Supply

The first, and perhaps most obvious benefit of grounded DC output is the safety protection element. Consider a wire running from the -V output wire to the earth ground system via a green wire. If the +V wire anywhere in the system becomes loose and touches a grounded DIN rail or operator station metal cabinet, it will immediately have an unrestricted path to ground, blowing the fuse or breaker. 

This element adds inherent safety to the system, ensuring that no operator accidentally comes into contact with that live voltage by touching the metal case near the failure point. 

A second benefit of grounding the DC system is that a large network of interconnected ground signals creates a very consistent reference point. If you are measuring voltage with a meter, and the voltage at a junction is changing because of interference, but the reference is not, this would be a very noisy signal. However, if the reference is also changing at the same time due to that same interference, the signals relative to each other will actually be quite stable. When the same interference is on both lines, this is referred to as common-mode signaling, and noise is measured when only one of those signals is measured without reference to the other.

Often, it is recommended to connect the DC - output to a bonded ground in order to reduce or eliminate this common-mode noise. 

It is also convenient for the sake of measurements when the reference for any DC component can be any metallic object nearby. If the system is ungrounded, you must have a wire or components providing a direct connection to the DC supply common nearby in order to make measurements.

However, listing these benefits doesn’t always mean that should be done. There are several reasons why it is useful to leave the system ungrounded as well.

To Connect a DC Power Supply to Ground Or Not?

If the question ‘should you connect a DC power supply to ground, or should you not?’ is posed, the answer is not a straightforward yes or no. In many cases, it will not cause any problems. In fact, it will afford greater safety to ground the common -v of the DC output. 

In cases where instrumentation isolation is important, it is a bad idea to establish that connection. 

Engineers have established various reasons for and against grounding DC systems, many based on specific scenarios and cases. These practices are usually perfectly acceptable and lead to a variety of opinions and solutions. There is no answer for all cases, so the best option is to understand the benefits and challenges and learn to become proficient at troubleshooting so that systems can be built as reliably as possible.