Hi There, I have a somewhat delicate design predicament in connection with the above.
There is an existing UPS installation in a UK data centre which is to be upgraded. In amongst a few other design elements this involves the calculation of dc cabling circuits from UPS to Battery Panels and Battery Panels to Battery Strings. The client scope is simply to replicate existing with any additional elements to be added / upgraded but on modelling such, utilising site specific details and BS7671 calc requirements, there appears to be cable undersizing shortfalls on both of the aforementioned existing dc cabling circuits.
When this is advised to the client they then approached the D&B installer of the original system and put simply the installer has said that with UPS systems, where the load is momentary and for a finite period, together with temperature rise factoring, alternate calculations can be provided which allow higher load currents to pass for designated short time periods. Such calculations are not derived from BS7671 as it cannot apply due to any current carrying capacity calculations being based upon continuous loading. They also note that grouping de-rating factors need not apply.
When I have asked the installer to substantiate their calculation view they have noted 2no. documents, namely IEC 60092-352 and a technical handbook document developed by Siemens Germany. My initial challenge is that the latter IEC document relates to electrical cabling systems on ships and the Siemens handbook was initially compiled in 1987 and was seemingly last updated in 1990, so it's over 35 yrs old. The handbook is not available via any Siemens website and it also heavily refers to German DIN VDE standards.
I cannot see how I can consider the installers calculations as valid as they are substantiated by an IEC document that clearly does not apply to a traditional building installation or a handbook which could be out of date and therefore no longer relevant.
I have researched the momentary load / finite period premise online, I can appreciate the premise, but whilst I can find some vague technical papers partly relating I can find nothing up-to-date to support such, nor am I aware of anything within BS7671 that allows such extended current carrying capacities based upon cables for shorter periods.
I would very much appreciate any opinion or guidance on the foregoing!
