Phase 6: Power Supply and Circuit Breakers

In this article we discuss the design of 24 VDC power supply and circuit breakers (index of design phases).



6-Power Supply and Circuit Breakers

The design of the power distribution of the control panel is an important aspect that can greatly influence the availability of the plant. Not always the necessary attention is given to this point.

Power supply

Let’s look first at the 24 VDC power supply needed in many control cabinets. Our recommendations are as follows:

-Perform a conservative calculation of the power required by applying a coherent simultaneity factor and adding at least 20% reserve.

-Analyze the voltage range allowed by 24 VDC consumers in order to correctly design the power supply.

-Analyze the behavior of the power supply throughout the operating temperature range. This information is provided by the manufacturer.

-Analyze the behavior of the power supply in the case of micro cuts in the input voltage, especially if we are facing unstable power networks.

-Use a redundant power supply configuration whenever possible. For this, different technologies and commercial modules exist that allow to use both P.S. in a balanced way.

-Analyze what type of 24 VDC loads we have and how we are going to distribute and protect the different power lines. It is necessary to take into account in the design if any of these loads can demand peaks of consumption that adversely affect the rest, if so it will be important either to consider it in the calculation of the power or use electronic circuit breakers with adjustable current limiting.

-Analyze the efficiency of the power supply because it is important both for the electrical consumption and for the heat dissipation inside the cabinet.

-Depending on the environment and the application, analyze whether it is necessary to include in the design some capacitor bank to avoid problems caused by micro-cuts in the supply, as well as if DC/DC converters are necessary to isolate zones from each other or uninterruptible power supply (UPS).

Power Distribution

The distribution of power and the selection of circuit breakers is not generally complicated. The most important thing is to define correctly the number, levels and protection current of each circuit breaker so that the protection and discrimination of each line are correct.

Design must minimize as much as possible common mode failures, i.e. there is no single fault that causes unintentional trip of two or more switches. This is especially important when designing a redundant system with redundant power supplies and separate protections on each channel. For example, it is not a good design if we use a single circuit breaker to protect both power supplies in a redundant system, or if we use a single C.B. to protect all power to field elements such as electro valves, etc. In any case, all this depends on design criteria and if the budget is sufficient.

Another aspect to consider in the design of the power distribution is the difference between AC and DC, since it is very common to use the same type of circuit breakers for everything without analyzing whether it is actually correct or not. The DC tripping curve is different with a factor of 1.3. Keep in mind that if we want to make a very efficient design an option are the adjustable electronic C.B. In certain applications, this may be necessary.

Link to next phase.