Redundancy concepts for the auxiliary voltage supply

Redundancy concepts for the auxiliary voltage supply

Be on the safe side

Consistently monitored, from the mains to the load.

Your advantages

  • A consistently redundant auxiliary voltage supply from the mains to every load
  • Redundancy monitoring through permanent checking of the input voltage, output current, and decoupling section
  • Clear messages via LED and signal contact
  • Long service life of loads, thanks to a constant voltage level
  • Long service life of power supply unit and DC/DC converter, through even load distribution


In many process engineering systems, availability is of paramount importance. If system parts or even individual components are briefly interrupted, this can lead to long and therefore costly production failures as a result of long shutdown or startup times of processes.

As such, in many cases, redundant systems are an effective method of avoiding the Single Point of Failure. This is also the case for the auxiliary voltage supply required everywhere, which is used in most fields with 24 V DC. To implement redundancy for the 24 V supply, two auxiliary voltage networks are switched in parallel and decoupled from one another using redundancy modules. The outgoing supply is distributed to the individual loads via corresponding fuse distributors.

If one takes a closer look at the common loads in the process industry, you will find DCS systems (Distribution Control System), remote I/O stations, and active marshalling distributors, which are often supplied by two power terminal blocks decoupled from one another. In addition, you will find numerous other loads such as signal conditioners, relays, 4-wire transmitters, which only have one voltage input.

Here, the following questions are posed:

  • How does the auxiliary voltage supply need to be structured in order to supply these two different load types and ensure high availability?
  • Is using two power supply units switched in parallel sufficient?
  • How is redundancy monitored?


Redundancy monitoring of QUINT ORING  

Redundancy monitoring increases availability

Depending on which redundancy concept you wish to implement, Phoenix Contact offers the perfect solution:

Redundant power supply network
When considering a redundant auxiliary voltage, the first question to be answered is whether power failure of the low-voltage network may result in failure of the control technology.

If this question is answered with no, then the auxiliary voltage network should be supplied from two different networks. Either by two independently supplied low-voltage systems or by a low-voltage system and, for example, a battery system.

Power supply unit redundancy

The two now obtained independent networks must be suitably distributed and combined together to the right locations.

The low-voltage networks are converted to the level of the auxiliary voltage networks using modern switched-mode power supply units in the switch rooms. In battery units, load fluctuations in long cable paths results in voltage fluctuations, which may impair the function and service life of loads.
As such, the voltage from the battery units should be stabilized to the desired voltage level by means of DC/DC converter prior to distribution and therefore prior to loading.

Examples for redundant power supply  

Supply from 2 low-voltage networks in front of decoupling modules (see figure to the left)
Supply from low-voltage network and battery network in front of decoupling modules (see figure to the right)

The current intensity and the position of the power supply units and DC/DC converters (and therefore the distance to the loads) play a key role when selecting the right voltage level and conductor cross sections.

Similarly to the battery unit, the following also applies here: the more centralized the conversion to the final auxiliary voltage, the greater the risk of voltage drops on the long cable paths
to the loads. 28 V DC is often applied, in order to make the required 24 V DC available on the load. In such cases, one often selects large conductor cross sections to minimize the voltage drop.

If the two redundant auxiliary voltage paths are then switched in parallel, they should be decoupled with suitable diodes in order to prevent compensating currents.

QUINT ORING diode modules  

The redundancy module signals OK

In doing so, it must be ensured throughout the entire system lifecycle, that redundancy is only present when the total load current of all loads is not greater than the maximum current of an individual power supply unit. This is the only way to ensure that in the event of failure of a path, the other one can fully take over its supply.

Intelligent diode modules (e.g., QUINT ORING) take over the monitoring function of the total current and output alarms if the current draw becomes too high. This facilitates extensions and identifies gradual errors (predictive maintenance). Furthermore, these intelligent modules also ensure even loads of both network paths through Active Current Balancing (ACB), which maximizes the service life of power supply units or DC/DC converters.

If a device drifts off on the output voltage side too much, this behavior is reported in good time. A fuse distributor often follows after the decoupling diode. From here onwards, the supply chain is no longer redundant, even if one supplies loads with redundant power terminal blocks via two different fuses. Errors occurring on the network or on the fuse distributor may still result in the failure of the system here.

Fully redundant auxiliary voltage supply

Connection of loads via decoupling modules  

Connection of loads via decoupling modules

The optimum redundancy concept consistently comprises two independent networks, which are connected in a cascaded manner via two power supply units (or DC/DC couplers) with two intelligent redundancy modules. This is the only way of actually supplying all loads redundantly, which evenly loads the individual auxiliary voltage networks
and monitors the redundancy.

For each load, two separate supply cables are pulled out: one from the first and one from the second potential distributor. In this way, now either directly connect the redundant power terminal blocks from type 1 load or directly in front of type 2 loads, then the two separate auxiliary voltage paths are combined to form a supply by means of another decoupling module.


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