Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Replies 7 replies
  • Subscribers 35 subscribers
  • Views 219 views
  • Users 0 members are here
Options
You may also be interested in

DC Cabling Current Carrying Capacity - Continuous vs Momentary / Finite Period Assessments Query

ISL

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by an IEC document that clearly does not apply to a traditional building installation or a handbook which could be out of date and therefore no longer relevant.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!

Parents
  • 0

    The concept of "thermally equivalent" currents is not unknown and is detailed in one of the GNs (I'd have to check which) - basically is is valid to have a "design current" (Ib) that's equivalent to the medium/long term effect of the load, rather than just using the peak current (we sort of take that approach anyway, as we often disregard starting currents). I believe that's all acceptable under BS 7671. The time periods aren't that long though (typically minutes for small cables, maybe tens of minutes for larger ones (I suppose you might just get into hours for very large c.s.a.s)) - so that may or may not be sufficient justification on it own.

    BS 7671's cable data is based on a large number of assumptions - and as it's "informative" rather than "normative" it's not strictly part of the standard and so other approaches are permitted. You could for instance decide that a shorter working lifespan for the insulation is acceptable, you could run the conductors hotter (it's interesting to look at the rating for automotive cabling - where the design lifetime is closer to 10 years rather than 70), or use different insulation that survives higher temperatures. That does have knock-on effects though - conductor resistance would be higher, so increased voltage drop, increased loop impedance, and the need for higher temperature terminals ... but that can often be dealt with.

       - Andy.

Reply
  • 0

    The concept of "thermally equivalent" currents is not unknown and is detailed in one of the GNs (I'd have to check which) - basically is is valid to have a "design current" (Ib) that's equivalent to the medium/long term effect of the load, rather than just using the peak current (we sort of take that approach anyway, as we often disregard starting currents). I believe that's all acceptable under BS 7671. The time periods aren't that long though (typically minutes for small cables, maybe tens of minutes for larger ones (I suppose you might just get into hours for very large c.s.a.s)) - so that may or may not be sufficient justification on it own.

    BS 7671's cable data is based on a large number of assumptions - and as it's "informative" rather than "normative" it's not strictly part of the standard and so other approaches are permitted. You could for instance decide that a shorter working lifespan for the insulation is acceptable, you could run the conductors hotter (it's interesting to look at the rating for automotive cabling - where the design lifetime is closer to 10 years rather than 70), or use different insulation that survives higher temperatures. That does have knock-on effects though - conductor resistance would be higher, so increased voltage drop, increased loop impedance, and the need for higher temperature terminals ... but that can often be dealt with.

       - Andy.

Children
  • 0 in reply to AJJewsbury

    Hi Andy,

    Thank you for your reply, most helpful.

    My main issue is that there is pressure being bought to bear by the client on the project to consider and accept the alternate calculation methodology, which I have no issue with in principle, but the substantiation they present to support their alternate calculation is 2x documents that I am struggling to readily accept.

    When I have calculated the UPS to Battery Panel (BP) dc cabling, protected by a 800A mccb, the post Ca/Cg adjusted It is 980A, the 2x 150mm 1C cables per pole can achieve an Iz of 864A therefore there is a notable shortfall. The alternate calc provided by others indicates that 2x 150mm 1C cables per pole can accommodate an Iz (or Ikb as they note it) of 1040A for a period up to 600s (10mins).

    The UPS systems are each 1200kvA / 1000kW, ultimately supported by 4x BP / Battery systems, which are to provide min. 10mins autonomy. The system will either be in a normal float charge state where the current apparent is minimal but the dc voltage is at its highest or under discharge 

    To further muddy matters there are a few other issues apparent that I have raised i.e. the primary mccb and string fuses in the BP are not fully rated to the potential dc voltage apparent, the string cable sets are also protected by 800A fuses but cabled via 1x 95mm 1C cables per pole, there is no clear discrimination between the devices in the BP, it goes on. All of this erodes my confidence in the original systems design and therefore makes me reluctant to continue to accommodate and match such with any newly added elements.

    Simon

  • 0 in reply to ISL
    ISL said:
    dc cabling, protected by a 800A mccb, the post Ca/Cg adjusted It is 980A, the 2x 150mm 1C cables per pole can achieve an Iz of 864A therefore there is a notable shortfall.

    I've only dealt with much smaller systems, but in those battery fuses (and CBs) are often selected to provide only fault protection - (overload protection either not being necessary due to the nature of the load (i.e. the inverter in the UPS in your case) or is dealt with downstream) - in such cases the fuse is often 2 or 3 times the expected design current, but the cables are still sized according to the actual current (Ib) rather than the protective device (In). If your actual currents are lower, there may be some margin there.

    10 mins is a reasonably short time for such large cables - it might be worth a quick back-of-an-envelope calculation to see what the adiabatic temperature rise would be (if you know the per metre resistance, and current, you can calculate the energy that goes to heat the conductor, then apply that to the heat capacity of copper to work out the temperature rise) -  that might prove to be either reassuring or justify your doubts.

    Discrimination between devices in series on the same circuit typically isn't a concern - if the end effect is the same (it all stops working) whichever device opens first, you're not really loosing anything. If one device opening in preference to another means that some loads, especially critical loads, could continue, that's a different matter. BS 7671 doesn't demand discrimination in all situations - just provides means of achieving it, if it's required. 

       - Andy.

Community Rules & Guidelines