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110v transformer feeding db

MrJack96


there is a DB board fed from a centre tapped transformer only on single pole breakers. The board feeds instrumentation. Is there any issue changing it to 110v and tie the neutral to earth there is no socket outlets fed on this so can’t see it as an issue?
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    Could you be more specific about this? Is the system currently on the output of a 115-0-115 transformer, centre-tapped earth, or is it a 55-0-55 centre-tapped earth?


    Do you know why the centre-tapped earth system was used in the first place? It could well be used for EMC, and therefore making it "single-ended" could cause issues with the instrumentation, in which case not a good idea.


    However, if a centre-tapped earth system is used, the breakers would usually be selected to disconnect both line conductors?
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    Sorry yeah it’s 55v 0v 55v and only a breaker on one 55v leg. Also what are the ZS values for 110V circuits I can’t find it in the regs?
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    Also what are the ZS values for 110V circuits I can’t find it in the regs?

    For centre-tapped 110V they're in table 41.6. For 0-110V ones you'd have to calculate them using the formula in 411.4.4 and the data from appendix 3 or the device manufacturer  (or just double  the table 41.6 figures).

       - Andy.
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    Be aware that if the exposed voltage is not considered enough to kill then the permitted breaking time is not  limited to the normal disconnection times of 0.4 secs or 0.2 secs, that are all about heartbeat  periods,  but instead is limited by things catching fire, so you may allow several seconds of breaking time.

    regards Mike.
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    It says 5s disconnection times  for the 55v centre tapped would it still be the same for 110v if I doubles the ZS values say a type b 10 amp is 1.05 ohms so 2.10 ohms I for a 110v to earth?
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    It would be, except that is for the same fault current and so gives the same disconnection time on the higher voltage. As the voltage rises the chance of surviving a shock fall, so the  exposure time has to be reduced - so we need the fault current to rise and the fault duration to fall.

    regards Mike.
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    It says 5s disconnection times  for the 55v centre tapped would it still be the same for 110v if I doubles the ZS values say a type b 10 amp is 1.05 ohms so 2.10 ohms I for a 110v to earth?

    Sounds about right to me.


    As a sanity check using the same method for 230V would give 1.05Ω / 55V * 230V which gives 4.39Ω which seems to about match Table 41.3's 4.37Ω for a B10 for 5s disconnection time at 230V (allowing for some rounding in the printed numbers). In all cases you just need to ensure the MCB sees enough current to trip within the required time - and that current is the same regardless of voltage - so you just need to adjust the resistances so you get the same current for differing voltages (Ohm's Law).

       - Andy.
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    As the voltage rises the chance of surviving a shock fall, so the exposure time has to be reduced - so we need the fault current to rise and the fault duration to fall.

    That's a good point - 110V isn't RLV (unlike 50-0-50) so 5s disconnection times don't generally apply - table 41.1 seems to suggest 0.8s would be needed on TN systems (for final circuit <=32A and socket circuits <=63A). As it happens that'll likely be same same Zs as calculated above for most MCBs, but won't be for fuses (or possibly D-type MCBs).


       - Andy.
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    MrJack96:


    there is a DB board fed from a centre tapped transformer only on single pole breakers. The board feeds instrumentation. Is there any issue changing it to 110v and tie the neutral to earth there is no socket outlets fed on this so can’t see it as an issue? 




    How can you tie the neutral to earth?


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    Have to get a 110v-0v transformer

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