Technical Article

Grounding DC Power Supplies

May 03, 2020 by David Peterson

Should you connect a DC power supply to ground, or not? The answer to this is not a straightforward yes or no. This article explores the benefits and drawbacks of grounding DC power supplies.

DC power supplies are a common — perhaps almost guaranteed — component of a control system, especially one containing a digital controller. An often-discussed matter of DC power systems is the connection of the output to the ground system interconnecting the 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 24V. Computer power supplies (including PLC power supply units, or PSUs) usually output 5V and +/- 12V, all at a constant, direct current polarity. When examining the output wires, they only contain a + and a - terminal and very rarely contain a protective earth (ground) connection. Yet for AC systems, this bonded ground system is regulated and required very strictly.

 

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

  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 Systems

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

The answer comes from the NEC section 250.162, referring to the grounding of two-wire DC systems, which includes the 5V and 24V outputs, depending on your case. The regulation sets a strict limit on the required grounding if the voltage is in excess of 60V. 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 a 24V DC system is no, the output is not required to be connected to ground.

 

Benefits of a Grounded DC Output

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 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.

Ofen, 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 common — a wire 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 will be a bad idea to establish that connection. 

There are a variety of reasons that engineers have established for and against grounding DC systems, and many are based on specific scenarios and cases. Usually, these practices are 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.