DC Cable Replacement
During the First of three annual Tegg Preventative maintenance visits to the London Headquarters of Telstra International, a multi-tenanted co-location Data Centre, infrared inspections identified a severe heat issue on several of the incoming cable glands at the main mechanical LV switchpanel.
The cables terminated onto a 3,200A incoming A.C.B. supplying the L.V. panel supporting the critical cooling plant for the entire site including all of the data suites.
Temperatures at the glands were in excess of 160C and the shrouds had been hardened by the excessive temperatures to the point that they had broken away from the glands.
The power analysis that is also a part of the TEGG service confirmed that the cables were working within their design parameters and that the heating was not caused by overloading of the circuit. Infrared analysis and voltage-drop tests of the terminals at the A.C.B. further confirmed that there were no connection issues at the terminations. The A.C.B. was also subjected the the Ultrasonic analysis that completes the service and this verified that there were no internal connection issues on the main terminals of the device.
Further harmonic analysis and power quality test were carried out that eliminated distortion as the cause. During these tests the currents flowing through the armouring of the cables was measured and found to be a great deal higher than would have been expected. It was subsequently discovered that the installation method of the cables was not in accordance with BS 7671 and that the individual single cores of the phase conductors had not been separated. Furthermore the cables had been terminated using standard cable glands at both ends of the circuit allowing circulating currents to flow along the armouring and through the glanded terminations which were not correctly tightened, causing the damage to the conductors and shrouds.
Remedial actions and successful solution:
A proposal to replace the cables following a new route, installed and terminated in accordance with BS 7671 and with separate earthing conductors was approved and implemented. All new containment and installation works took place prior to a planned shutdown with no interruption to the operation of the site or its customers. The final terminations were made during the shutdown while the system was supported by the standby generators.
Following the final connections all damaged cabling and existing containment were removed from site. During this stage of the works the extent of the damage and the potentially disastrous outcome became apparent.
The phase conductors had become so hot during service that their insulation had melted together and they had to be cut into manageable lengths and removed as one.
The aluminium glands had fused to the gland plate and could not be removed from the plate
The inner insulation of the conductors where it passed through the glands had been so badly damaged that when the conductors were removed the burnt insulation cracked off to reveal the bare copper.