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Let’s paint a quick scenario. Preso Limited has this very large machine plant running an industrial recycling system. Its structure is such that any momentary network failure can cause the supposed managed waste to be disposed improperly into a waterbody nearby. We can’t have that damaging the reputation that Preso has managed to keep as an environmentally friendly cooperation now can we? This is where Redundancy steps in.

Redundancy in layman’s terms means back-up in a case of failure of any kind. With respect to PLCs, Redundancy refers to the use of a stand-by component or system in fault condition. This duplicate component should have a stake-hold in your process and have the ability to stop the entire process. This is because it is not exactly economic friendly to have a back-up for all components in themselves. In this light, this article would explain the types of redundancies with respect to the components and severity.


A) By Components
1. PLC Redundancy:

This is a case of having a redundant PLC as a back-up to avoid data loss and abrupt stop of the field devices whenever the primary PLC fails.

2. Cabling Redundancy:

This is a case of having 2 alternate communication paths if the primary path fails. The diagram below shows a ring network where each ethernet switch has at least one primary path and one redundant. For devices in a PROFINET network, the MRP (Media Redundancy Protocol) is used while for the Ethernet/IP network devices, DLR (Device Level Ring) is used. They both have the ability to detect interruption and switch to the alternative path.

3. Server Redundancy:

This is a case of having a primary monitoring server and a secondary one.

B) By Severity
Cold Redundancy:

In this case, the total shutdown of the system or process downtime is not as critical as Preso Limited’s recycling system, therefore is allowed. Most times, the help of an operator or human intervention is needed to switch over from one component to the other.

Warm Redundancy:

Moving a step stricter from cold redundancy, a long process downtime is not acceptable in this redundancy type, rather a brief or momentary outage can be considered. In an ideal situation, there should be some link between the primary processor and the redundant one so that the both processors can be abreast with what’s going on in the system and the redundant processor can continue from where the primary stops in the face of failure. In the case of a warm redundant system, the primary PLC updates the redundant PLC periodically, thereby allowing the redundant PLC take some time to catch up and take its place without loss of data if the primary develops some fault.

Hot Redundancy:

Preso Limited’s recycling system is a perfect example of a system that should employ Hot Redundancy. This case of redundancy does not give room for any form of outage or downtime in its process. Whenever a failure occurs, an automatic switch happens immediately to the redundant component. Also, the primary and redundant PLCs are updated at almost the same time and speed of the system’s state and condition so that when a switch is made, no time is lost in coordinating the data in the redundant PLC before it can start its operation.

As an automation engineer, because of the severity of some projects, the knowledge of the exact type of redundancy to employ is very important, else the repercussions could be counting in millions of money and even lives lost.