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High speed, low voltage inspection at London Stansted

22 July 2013

London Stansted Airport is open for business 24/7 and reliable and continuous operations depend on the safe and efficient functioning of its electrical networks. This article from IRISS shows how new inspection window technology enabled airport engineers to extend the scope of electrical equipment inspections, potentially improving both system resilience and safety.

Stansted is London's third busiest airport and needs to be at the ready for every eventuality, whether the cause is bad weather or a security alert. Some 17.5 million people currently pass through the terminal building at Stansted each year. 

And although the early hours of the morning provide a relatively quiet period for planned maintenance, passenger processing normally starts around 3.30am, by which time all electrical systems must be running. Historically, this put the maintenance teams, responsible for the reliability of all low voltage equipment, under huge time pressure.

Too much to do, too little time

At best, engineers had four hours per night in which to conduct predictive maintenance inspections and by the time they had made the system safe, this window of opportunity was reduced even more. This meant the entire inspection cycle was significantly protracted and no system could be checked under load.

David Potter, an engineer with 24 years experience at Stansted, is responsible for strategic planning and maintenance of electrical distribution on site, both high and low voltage. But whilst parts of the high voltage network can selectively shut down without compromising the operation, the low voltage equipment does not have similar capacity.  

“Our high voltage network is owned and managed by UKPNS but maintenance of low voltage equipment is down to our own engineering teams,” he said. “We agree the maintenance schedule with the Airline Operators Committee three months in advance and our Maximo planning system flags up what needs to be inspected each night, but in every case it was a race against time. Nevertheless, this work was vital.” 

The electrical infrastructure at London Stansted Airport is huge. Power is brought onto the site at 33kV and transformed down to 11V. The low voltage network feeds all manner of systems in the main terminal building, typically IT and baggage handling. So any unexpected interruption in the power supply could have serious consequences.  

The potential cost of failure is also huge. The Service Quality Rebate scheme (SQR) stipulates that the airport has to pay compensation to its customers, the airlines and handling agents, if it fails to meet an agreed level of service.  

The largest size windows enable whole busbars to be inspected
The largest size windows enable whole busbars to be inspected

Although efficient maintenance had so far prevented serious low voltage system failures, Potter wanted greater assurance. He had considered the introduction of infra-red windows to complement outsourced thermal imaging surveys and to allow live inspection, but all available products on the market were deemed unsuitable.  

He explained: “A small crystal window may have been fine for small switchgear but the size of our equipment meant that we would end up with so many windows, our substation would look like a submarine. Even if it had been viable from a practical standpoint, the cost would be huge. And crucially an individual window would be too small to allow the all important inspection of an entire busbar.”

The transmissive polymer solution

Potter finally found a solution after reading an article in a trade magazine on infrared windows made from transmissive polymer.This fully impact-resistant material allows the window to be any size, which is perfect for applications involving large switchgear.  

This technology, supplied by IRISS, was installed in substations serving the terminal block in under two months, and Potter said potential savings in inspection time and the ability to inspect live systems, including the busbars, easily justified the investment.  

The specification was approved on 6th November 2012, a comprehensive installation plan was then drawn up by IRISS and the entire job completed by Christmas. Having installed an isolation sheet on each cabinet, the thick door panel was removed to the access road where the required hole was cut with a jigsaw and the appropriate IRISS CAP Series window fitted. 

This work continued until a total of 72 windows had been installed, comprising a combination 6”, 12” and 24” products all available in the standard range. The largest window is ideal for the thermal inspection of busbars and multiple components.

IRISS CAP series inspection windows
IRISS CAP series inspection windows

The CAP product can be fully customised to suit the application, meaning a window no longer needs to be round, nor is its size restricted. A crystal window by comparison becomes too fragile beyond a 4” diameter. IRISS says this technology reduces the number of windows required, their installation time and associated costs.

Following the installation of the CAP Series windows at Stansted, the impact on inspection time was immediate. IRISS carried out the first thermal imaging survey using the new infrared windows on 8th January this year and only two minor faults were discovered.

“It took just five hours to complete the job in daylight hours. That also included walking from one substation to the next,” Potter confirmed. “Previously it would take us two nights to inspect just one panel. Now there’s no need for isolations or back feed and our engineers’ safety is completely assured.  A single thermographer with a thermal imaging camera and without PPE can do everything.”

First ever benchmark

The resultant thermal inspection report from IRISS provided a comprehensive snap shot of the health of the entire low voltage network at London Stansted Airport. 

Potter added: “I now have the thermal performance of each system printed out in this report and, for the first time, a benchmark on which to base future thermal inspections. Previously, it would have taken us a couple of years to inspect the whole complete network and even then we had no idea what was happening under load.” 

“All we could say is that everything had been thoroughly inspected.  Now we can see the live circuits and cables and the temperature rise on busbars,” he continued.  “This means we are able to apply trends to every system. In short, I now know the terminal is truly safe. I’m really sold on this technology!”

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