Discussion in another thread has made me think of something that has never occurred to me before.
In order to ensure that ADS will happen as intended in response to a ‘negligible impedance’ (i.e. taken as zero for calculations) L-CPC fault, we divide “the voltage” by the measured total loop impedance (Zs) to get the fault current, and then compare this with the characteristics of the OPD which, in the case of a Type B MCB, we assume that, in the ‘worst case’, could require a fault current of 5 x In.
Traditionally, “the voltage” we used for this calculation was the nominal supply voltage (230V), but this would mean that magnetic tripping would not occur if the Zs were right on the limit and “the voltage” were less than 230V. Accordingly, as we know, Amd3 of BS7671:2008 introduced the concept of “Cmin” (with a value of 0.95), with the effect that the calculations were done on the basis that “the voltage” was 218.5V - thereby decreasing the specified ‘maximum Zs’ figures.
However, I’ve never previously stopped to think about what “voltage” is relevant, subconsciously probably ‘assuming’ that it is the voltage supplied to the installation in question. However, I’ve just realised that that is wrong ....
The ‘fault loop’ we are interested in, whose total impedance is Zs, is the entire loop from the final circuit in the installation all the way back to the transformer at the substation. The voltage driving current around that loop is the voltage at the secondary of the transformer, so the fault current will be equal to that voltage (at the transformer) divided by the Zs (total impedance) of the circuit concerned - regardless of what ‘supply voltage’ is being delivered to the consumer installation in question.
The supply voltage to premises is allowed to be anything in the range 216.2V to 253V. However, if/when the voltage of supply seen by a customer is fairly low in that range, it is almost invariably due to voltage drop in the LV distribution network between the transformer and the consumer, not due to a very low voltage at the transformer. I find it very hard to believe that the voltage at the transformer is ever (under normal circumstances) less than 230V, and would suspect that it would rarely even be below 240V.
That being the case, unless I’m missing something or thinking all wrong, could it not be said that the regulations are now being almost ridiculously cautious/ conservative in implicitly requiring that ADS will still ‘work’ when the voltage at the transformer is as low as 218.5V - particularly given that that would almost certainly mean that many/most of the customers being supplied from that transformer would be receiving a supply below the ‘minimum permissible’ 216.2V ???
Kind Regards, John
In order to ensure that ADS will happen as intended in response to a ‘negligible impedance’ (i.e. taken as zero for calculations) L-CPC fault, we divide “the voltage” by the measured total loop impedance (Zs) to get the fault current, and then compare this with the characteristics of the OPD which, in the case of a Type B MCB, we assume that, in the ‘worst case’, could require a fault current of 5 x In.
Traditionally, “the voltage” we used for this calculation was the nominal supply voltage (230V), but this would mean that magnetic tripping would not occur if the Zs were right on the limit and “the voltage” were less than 230V. Accordingly, as we know, Amd3 of BS7671:2008 introduced the concept of “Cmin” (with a value of 0.95), with the effect that the calculations were done on the basis that “the voltage” was 218.5V - thereby decreasing the specified ‘maximum Zs’ figures.
However, I’ve never previously stopped to think about what “voltage” is relevant, subconsciously probably ‘assuming’ that it is the voltage supplied to the installation in question. However, I’ve just realised that that is wrong ....
The ‘fault loop’ we are interested in, whose total impedance is Zs, is the entire loop from the final circuit in the installation all the way back to the transformer at the substation. The voltage driving current around that loop is the voltage at the secondary of the transformer, so the fault current will be equal to that voltage (at the transformer) divided by the Zs (total impedance) of the circuit concerned - regardless of what ‘supply voltage’ is being delivered to the consumer installation in question.
The supply voltage to premises is allowed to be anything in the range 216.2V to 253V. However, if/when the voltage of supply seen by a customer is fairly low in that range, it is almost invariably due to voltage drop in the LV distribution network between the transformer and the consumer, not due to a very low voltage at the transformer. I find it very hard to believe that the voltage at the transformer is ever (under normal circumstances) less than 230V, and would suspect that it would rarely even be below 240V.
That being the case, unless I’m missing something or thinking all wrong, could it not be said that the regulations are now being almost ridiculously cautious/ conservative in implicitly requiring that ADS will still ‘work’ when the voltage at the transformer is as low as 218.5V - particularly given that that would almost certainly mean that many/most of the customers being supplied from that transformer would be receiving a supply below the ‘minimum permissible’ 216.2V ???
Kind Regards, John