Solving clients’ problems
We measure key parameters such as leakage currents, neutral-
A power survey can be performed on its own or as part of a full Data Centre Environmental audit.
While monitoring power we measured an unnecessarily high value of 249.2v (maximum allowable 253.0v) from the UPS. This was subsequently adjusted down. The load indicated that the UPS was significantly out of balance and we also noted that red phase carried twice the load of the other two. Red phase also exhibited the lowest voltage and worst harmonic distortion. In each case, the higher the indicated load, the worst were both output voltage and harmonic distortion for that phase.
Most IT manufacturers recommend that harmonic distortion (THD) does not exceed 5% on any phase. Consequences of not observing this are reduced reliability and service life of equipment. It is notable that while some major manufacturers do allow 9% THD, they limit the third harmonic to 7% which is still exceeded here on red and yellow phases. We recommended that the client confirm that the capacity of the UPS is not being exceeded, that they confirm the accuracy of the values of current being reported and then balance loads across all three phases, that they understand the reason for a low red phase voltage if it still exists, and correct as appropriate and that they confirm that harmonics fall to below 5% on all phases.
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Data from L1 and L3 phase voltages recorded over a 24 hour monitoring period show that while L3 phase responded in a stable way, during periods of high activity numerous sags were noted on L1 phase, also causing a significant drop in average voltage, the white line. The problem was traced to poor phase load balance with the L1 breaker operating at its rated limit and 'chattering' under excessive load. We recommended that the system be rebalanced and the breakers uprated at a suitable opportunity.
Our audit revealed a 7A leakage current from a PDU, initially identified at the SWA feed exiting the UPS output panel in the LV room. Typical values seen elsewhere on the site were less than 0.5A. Possible causes would be a miswired commando plug, a failing power supply or a shorted neutral. The consequences are potentially dangerous currents on earth cables, and radiated energy in Cat5 cables. The latter may disrupt data transmission, and slow systems because extra error correction is applied to address corrupted data. We investigated and pinpointed the affected cabinet. The miswiring was corrected during a suitable maintenance window. We were also able to confirm the location by tracing radiation emitted from Cat5 cables, before power was interrupted to effect correction.
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