Upgrading legacy alarms

16 November 2021

Ian Loudon explains how Uganda’s largest generator of electricity was able to upgrade its safety-critical alarm systems without having to overhaul the entire site infrastructure.



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To alert operators of an event threatening the safety of assets, people or the environment, industrial sites rely on alarm annunciators – directly hard-wired into various processes. When an event is detected, the relevant window on the annunciator panel lights up and sounds throughout a site, notifying operators to act quickly. 

For Eskom – which operates hydroelectric plants in Uganda – this event could be a generator component failure which, if not caught immediately, could result in costly damage to plant assets.

But, in a digital age, where organisations heavily on computer-based programming, do we still need bulky, hard-wired annunciator panels? 

The honest answer is yes. It’s true that PC-based systems provide efficient monitoring and reporting across a range of communication points in an industrial setting. However, we often see companies overloading their systems with complex visualisations, making it difficult to identify and act on critical alarms when they arise.

Alarm annunciators are therefore still critical for operators, allowing them to identify and act on an event before any serious damage is caused. But what happens when the original annunciator manufacturer can no longer supply spare parts or support you when you need to upgrade your system?  

Fit, form, function
The problem we are finding is that many alarm annunciators still in operation today today were installed decades ago and do not meet the current IEC 61508 safety integrity levels (SIL-1). What’s more, former alarm system providers, like Rochester, Highland and Bristol Babcock, no longer exist to offer system support. This is why ESKOM enlisted Omniflex to replace its obsolete legacy Highland Rochester MPAS90 49-point system.

In the case of the hydroelectricity stations, the new Omni16R annunciator system was retrofitted into the existing panel space. The IEC standard 3U rack was factory engineered to match that of the legacy annunciator, so that it was bench-ready tested before installation on site. This allowed staff at the facility to make the retrofit without the need for specialist tools or engineers and the existing wiring were matched with the terminals of the new unit. The old 50Vdc powered energy-intensive incandescent lamp alarm displays were replaced with modern energy efficient high brightness LED technology.

The biggest challenge for any company looking to upgrade its alarm system is doing it without disrupting existing cabling. Eskom requested that all annunciators be wired to the existing 50V dc wetted field contacts and this was achieved using 16 channel isolated terminal boards, connected via ribbon cables to the rack. The team was able to engineer this off-site to help reduce installation time and disruption to plant productivity.

The process of retrofitting an alarm system to fit, form and function properly is relatively straightforward. But the importance of digitalising alarms cannot be understated. 

As well as giving local visual and audible indications to notify operators of an abnormal occurrence, each annunciator can be connected as a Modbus slave device on an RS485 network. This means all alarms can be remotely monitored, either on a DCS or a local SCADA system, and can provide historical pre and post alarm data needed for auditing and fault diagnostic purposes. 

As Uganda’s largest electricity generator, there’s a lot at stake for Eskom’s power plants. Retrofitting a modern system, with capacity for remote monitoring, enables greater operator efficiency and improved alarm management, which all contributes to an efficient running plant with the additional benefit of having Key Performance Indicators (KPIs) which offer a measurable value to demonstrate how effectively the plants are meeting their objectives.

Ian Loudon is international sales and marketing manager at Omniflex.


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