Power Quality Case Study 1 - Mercury
POWER QUALITY CASE 1
COPPER DEVELOPMENT ASSOCIATION
Publication No 134
MERCURY SOLVE THREAT OF INTERFERENCE
Best practice preventative maintenance and the importance of circuit separation.
The case illustrates the need for companies to consider electrical installations as an integral part of site operations, and to ensure circuits are designed to minimise interference between essential and non-essential loads.
As might be expected, Mercury Communications take the integrity of the infrastructure within their major network sites very seriously. In addition, to the routine preventative maintenance, common in professional engineering departments, staff are required to identify, in advance, any technical threat to the operation of their telecommunications equipment.
For electrical engineering staff at the headquarters site at Bracknell, this means regular inspection of the quality of on-site power supply, in particular supply to operation-critical equipment.
It was during one of these inspections that the case outlined here was identified.
As part of an office refurbishment and upgrade, Mercury introduced a new Penta desk system into the customer service centre. A plan diagram of a Penta desk is shown in Figure 1. These desk systems are designed for ergonomic and space optimisation and have proved very popular amongst Mercury staff.
Groups of desks are fed from a "clean" supply (200KVA UPS with generator back-up) to distribution boards on each floor. Individual desk systems are then supplied by armoured cable to distribution boards mounted in the centre of each desk arrangement.
Fed from this panel are 13A socket outlets at each position in the desk, and the integral fans and lights.
At first sight this set-up appears simple and efficient, but it was in Autumn 1995 that electrical engineering staff began to identify a potential problem.
The problem with this set-up is that the clean supply is "contaminated" with non-operation-critical load. The vital PCs are on the same local circuitry as non-essential laser printers, fans, lights, indeed any load which might be plugged into one of the 13A sockets, including vacuum cleaners.
Measurements taken at the Penta desks using state-of-the-art Fluke Equipment revealed some disturbing effects, namely voltage dips caused by fan switching and voltage waveform "flat-topping" caused by laser printer switching. Figure 2 shows an example of the latter.
This situation poses two significant threats to operational security in the customer service centre:
· The disturbance to the voltage waveform is sufficient to interfere with the PCs;
· The operation time of the UPS in the event of failure will be severely reduced due to it serving a high proportion of energy-intensive, non-critical load.
Mercury decided that the required solution was a comprehensive upgrade of the circuitry, to separate essential from non-essential load at the Penta desk units.
An initial area of 14 Penta desks occupying 1315m2 was selected for the upgrade, the basic material requirements being:
· 3 extra distribution boards;
· 12 extra ring circuits between 70m and 142m in length;
· 97 extra floor boxes;
· 12 spurs at 3m per spur.
The work was done in the ‘slow periods’ of late evening and weekend, which though more costly, minimised disruption to the operation of the customer service centre.
Certainly, it would have been cheaper to do the work at the time the Penta desks were installed, but the important point is that the circuit separation was finally performed and the threat to operational security removed.
One interesting feature of the upgrade was the additional copper used - the improved design of the final circuit power distribution system contained around 25kg of copper per 100m2 compared with only 19kg per 100m2 previously - an increase of around 30%.
Such increases in copper consumption are inevitable when best practice circuit separation is undertaken. For the price of a few hundred metres of copper cable, power distribution systems can be designed to guarantee minimal interference between essential and non-essential loads, thus safeguarding against disruption to vital business processes.
Some important conclusions can be drawn from this case:
1. Professional electrical engineering departments are proactive in preventing operational threats becoming reality.
2. When facilities are upgraded, it is important that the electrical installation is reviewed at the same time, and upgraded if necessary.
3. A best practice power distribution system must contain sufficient final circuits to service operation-critical and non-essential loads separately. Such a system is likely to contain around 30% more copper cable than standard designs.
For further information on copper solutions to your power quality problems, see chapter 3 of publication 123 ‘Electrical Design – A Good Practice Guide’.
