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Afraid not.... fully loaded: ASHP, 32a Induction Hob, 2 x ovens etcNo chance of using a BS88-2 63A fuse?
and 4000sq ft so pretty big place!
Discuss High Ze causing headache ! in the UK Electrical Forum area at ElectriciansForums.net
Afraid not.... fully loaded: ASHP, 32a Induction Hob, 2 x ovens etcNo chance of using a BS88-2 63A fuse?
I am going to try our other ones... we've got couple of other Meggers - be interesting to seeI've have access to three different MFT, never got a matching Ze reading from any combination of them. One (Fluke) is always higher than the other two (Megger) with it not unusual to see a difference of up to 0.2 between the Meggers and the Fluke.
The 5s is somewhat arbitrary and related to the ADS regs. In reality the issue is as you say, the CSA becomes larger than necessary as it is not accounting for heat loss (i.e. it is not really adiabatic). So it is a safe option, but a sanity check is if the computed CSA is larger than the necessary line conductor to match the CCC limit of the OCPD then you really have gone beyond the limit!It was on the IET forum about why we should not use adiabatic for fault currents operating devices <0.1S or >5S. I couldn't remember but just found it again on there.
Going over 5S was more straightforward. Because the adiabatic is simplified it does not allow for heat loss, this results in artificially high csa's
Sort of. What you see in the time/current plots is typically known as "virtual time" and it is the equivalent time at that PFC leading to the same I2t let-through. In reality, and especially for fuses that are quite energy-limiting, the time on the plot can be uses with tolerable accuracy even allowing for varying phase angle of the fault.Going under 0.1s was more complex. 0.1S is 5 cycles of the supply waveform and going below that could give rise to greater currents than calculated due to the waveform being asymmetric and becoming distorted.
I would normally avoid relying on RCD if reasonably possible, but as you say where the only sane technical solution is RCD then it is perfectly good.I know a lot of people feel designing a circuit on a TN system and relying on a rcd for fault protection is a bad design but this is twaddle and this thread is a good example, a high Ze but even so it is still acceptable.
New supply in from DNO is PME, but we have Ze of 0.51
We’ve run a SWA sub-main across to house DB 25metres away, but struggling on suitable overcurrent protection. Even at 80amps BS 88…. Max EFLI 0.40!
Ok so I may have a case then..... assuming DNO have to comply with ESQCR? (not something I'm familiar with being honest)Actually there is something more fundamentally wrong here. If it is TN-C-S then, unless there is something very bad at the cut out, the supply Ze is the same as the supply impedance. With 0.51 ohms and 80A you would be seeing 40.5V drop in the incoming supply! That is more than the ESQCR limits of +10% -6% on 230V
Are you sure it is actually rated above 60A anyway? I think more discussion with the DNO is needed.
That what I said earlier confume with them before you do any thing else, before you take another route.Are you sure it is actually rated above 60A anyway? I think more discussion with the DNO is needed.
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