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By any chance, would you know the max Zs for this 15 amp MCB?
By any chance, would you know the max Zs for this 15 amp MCB?
Looking at the data sheet, and applying the UK regulation's approach but with USA value, it would be:
- Max current for 0.4s trip time = 30 * In (slightly less, but that is marked on horizontal scale) so that = 30 * 15A = 450A
- Assuming you are looking at the USA supply of 277V nominal line-neutral and that is +/-10% then the lowest supply is 277 * 0.9 = 249V
- So max Zs = 249V / 450A = 0.55 ohms
- As for the table I did earlier, I am not applying the typical 0.8 factor for hot cable resistance compared to cold design/test values.
Realistically if you are faced with that sort of a breaker curve I would be looking at putting in a high quality RCD of 300mA or even up to 5A trip point as it would deliver far faster disconnection in the event of an earth fault, not having inrush trip risk, and allowing a sane cable size based on 5% VD of something like #5 AWG copper.
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As mentioned before, two RCD in series could be used for "no single point of failure" and even at the $1k MCCB style pricing it is going to work out cheaper than that extra copper (and the time & effort to install and terminate it).
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Pure awesome! Though I would use -5% instead of -10% but it works either way.
Yup, I noticed that two. US breakers keep giving me a much lower Zs which kept throwing me off. Still confuses me to be honest. Basically most US breakers are a type E of F, if such a category existed after type D lol.
3/0 is a massive amount of copper. I'm basically calculating voltage rings around the post in hopes of being able to get away with a longer disconnecting time based on lower hand to foot voltage than 138 volts:
For your calcs, are you using 30*C or 75*C for the AWG wire system?
The UK voltage tolerance is +10% / -6% but the calculations are actually on 0.95 so -5% of nominal. However Aus/NZ seems to use the nominal cases so is more "optimistic".
What is actually applied would depend on local code/regulations but I have no idea of the details of USA policy.
That seems to be the case, though to be fair the EU range of MCCB also come with that style (basically fixed magnetic trip point for the family, but different thermal curves) as well as ones more like MCB with the "instant" magnetic point being at a fixed ratio to 'In'.
As well as the fancy but expensive ones with adjustable electronic trip up to the very high energy-limiting magnetic "last resort" trip.
With local rods you have no real hope of safe disconnection or protection, unless very deep and wet. Having a CPC is far safer but adds cost, though if you can use SWA armour and still disconnect it is not really making a difference to project cable cost (assuming armoured is used).
But from above it looks like an RCD is needed in any case to deal with that sort of length, even though it is not an RCD for direct-touch shock itself.
I assume 30C (i.e. cold, as measured), just got the AWG size & resistance from Wikipedia!
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I used this:
American wire gauge - Wikipedia
Looking at details it says 68F = 20C which is room temperature around my part of the world.
Also probably they are DC, starts to become a source of error for large conductors as skin effect kicks in.
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- Sep 5, 2010
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if digging in for 1 lamp post, why not fix 2 and stickk a propeller on the other one. wind power.
For large conductor yes, but its only about 20%-ish for 1000mm^2 so usually can be ignored. But also the inductive term starts to become more significant relative to the DC R1+R2, but even that is only about 10%-ish for 150mm cables.
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I used this:
American wire gauge - Wikipedia
en.wikipedia.org
Looking at details it says 68F = 20C which is room temperature around my part of the world.
Also probably they are DC, starts to become a source of error for large conductors as skin effect kicks in.
I should know about this link! ????
Anyway, do you know of a table taking skin effect into account? For a while I've desperatly been searching for an AC resistance at 30*C.
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At AC power frequencies the skin depth in copper is about 9mm, so any conductor that is less than around 20mm diameter it has quite a small effect. Somewhere there will be equations to allow its computation for different conductors but most folks in engineering take the easy route of getting a feel for when you can ignore something, and ignoring it.
For example it is unlikely you will know any AC system parameter to better than 5% uncertainty, let alone 1%, so once effects get down to the 5%-ish region they can be often ignored for most purposes.
The temperature coefficient for copper is around 0.00393 so each 10C increase in temperature has a 3.93% increase in resistance. Which is about 4%. Which is why the UK regs have a 0.8 factor for going from measured at 20C to Zs value working at 70C.
Yes, our regs do have detailed formulae for non-standard cases such as measurement very cold/hot and/or operation at unusually high temperatures, but that vast majority of industrial cases are just like commercial and domestic in the 20C test & 70C assumed max operation is perfectly applicable.
In terms of generic AC values we have tables in the regulations with the values to assume. For example this if for single cables in various configurations giving both the DC and AC "resistance" values (more correctly the voltage drop due to cable impedance):
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No, the UK regulations generally do not specify how you achieve disconnection, only the times, etc, needed for safety. But they do list approved device standards for doing so (e.g. BS / EN ones on fuses, MCB, RCD, etc).
That section you quoted (411.3.2.5) has been updated but really it was a clause to deal with cases where the usual fault disconnection (OCPD or RCD) are not feasible so other means of protecting against shock need to be considered (supplementary bonding, etc).
The note about "disconnection may be required for reasons other than protection against electric shock" is (I think) to point out you might need it for overload/fire protection anyway, even if that fails to meet the maximum disconnection times discussed in that section about shock protection.
But beyond that, there are other rules permitting no OCPD or special cases when disconnection might be more dangerous, e.g. support electromagnet in factory, etc.
If you look closely at that table you can see where the skin effect comes in by comparing the DC volt drop with the resistive term 'r' in column 5, but more generally it is the total impedance 'z' that would be used for volt drop if dominated by the cable.
Though close to a transformer, etc, where reactance is also a big factor you would be separately adding the r & x terms and finally computing z = sqrt(r^2 + x^2) to get your impedance.
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