Ensuring redundancy in critical systems

Probably no other issue generates more discussion or confusion in the electrical market than how to correctly implement a redundant power supply system.

Yet, redundancy is paramount when extreme reliability is required in power supply systems, as is the case in the DC supply to a DCS or PLC control system.

Redundancy is simply the duplication of critical components to ensure the continued supply of power in the event of a fault in any environment or application, regardless of whether it is an industrial plant, mining site or hospital.

To ensure the continued flow of power, dual redundancy is often implemented. This allows for a single component to fail. In critical applications such as safety shut down systems, triple redundancy is often used.

In this case, the probability of two concurrent failures occurring is so low that it would be almost inconceivable to have a total failure.

According to electrical connectivity supplier, Weidmüller, the design of a dual redundant power supply system should ensure that the failure of one power supply does not interfere with the operation of the other supply. Only when this is accomplished is redundancy attained.

Contrary to popular thought, a simple parallel connection of the outputs of two power supplies does not result in redundancy. A power failure does not always ‘fail to open

circuit’ and therefore, the failure of one power supply can place an additional load on the output of the second system, causing it to overload and shut down.

Each system needs to be isolated to ensure redundancy.

Diodes, sometimes called auctioneering diodes, incorporated in the design, will fully isolate each system and guarantee redundancy. Externally connected, full current-rated power diodes will establish a one-way current path from the output of both power supplies.

Fitted in series with the positive output of each supply, the diodes prevent the back-feed of current to the failed power supply and hence, the possibility of overloading the second system.

How a diode module can be incorporated into the design of a power supply system to achieve redundancy.

By using external diodes with two identical power systems, redundancy is achieved regardless of the type of power supplies selected.

If diodes are not fitted a further problem can occur. If one power supply fails, the second supply will almost certainly maintain power to the indicating light LED on the failed supply and any output failure alarms. The operator will falsely believe that both supply systems are still operational.

To ensure redundancy when incorporating diodes into power systems, users should check that the mean time between failure (MTBF) of the diode module is equal to or greater than the power supplies, otherwise the system MTBF will be compromised.

According to David Head, the company’s principal engineering consultant, it is also good practice to adjust the output voltage of each power supply so that they are as close as possible. As a rule, the diodes or diode modules should be continually rated to the full current of the power supply.

Said Head: “Guaranteed redundancy and peace of mind can be achieved with the use of diodes, regardless of which manufacturers electrical components are being used. Without diodes there is simply no guarantee of redundancy.”