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Hey all im just trying to learn max zs values from a NICEIC chart i have, now learning these values isnt a problem but the sections reffering to bs3036 fuses and type D mcbs both have two lists of readings - one each for 0.4s and 5s trip times - where as the sections for type B and C only have single lists. Could anyone tell me why this is pls?
Ive attached the chart im using
Thanks in advance
 

Attachments

Wilko

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Hi - short explanation - once the trip current is reached those mcb and rcbo trip within 100ms.
23A1DBAE-A056-42A4-81B5-36C2E345ACE8.jpeg
 

pc1966

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Hey all im just trying to learn max zs values from a NICEIC chart i have, now learning these values isnt a problem but the sections reffering to bs3036 fuses and type D mcbs both have two lists of readings - one each for 0.4s and 5s trip times - where as the sections for type B and C only have single lists. Could anyone tell me why this is pls?
First thing is you don't learn them - that sort of table is something you should have on-hand for any time you are planning a new circuit or testing an existing circuit:
  • The first point of reference is the "Big Blue book" as already said, i.e. the current 18th IET wiring regulations. That has various tables for fuses and breakers.
  • More likely you would have a copy of the IET On-Site Guide (OSG) with you. It has the more common things you need to know.
  • Finally for unusual cases you need to look at the manufacturer's data sheets. Most likely for a MCCB (moulded case circuit breaker) where they don't have a simple set of characteristics like the B/C/D curve MCBs you see in domestic electrics.
Also mentioned already is the reasons for the two values. In fact, there are two different (though related) things here:
  • The disconnection time for shock protection varies by circuit type, the two common values are 0.4s and 5s (see the BBB for the categories of circuit, but generally 5s on a sub-main and 0.4s final covers most cases)
  • MCBs have two distinct regions of operations. But B/C types don't have any time distinction between 0.4s and 5s, while D-curve (and fuses) do
Again (too slow today!) the typical characteristics of a MCB have been posted by Wilko and what it shows are two regions of operation for a MCB:
  • Inverse-time thermal trip (up to 3-5 * In for B-curve shown)
  • Instantaneous magnetic trip (above this current)
The thermal trip attempts to model a cable, etc, so the trip time decreases as overload increases so the heating of a typical cable is acceptable. Then above a certain point the magnetic trip kicks in and the breaker goes in tens of milliseconds or less, this greatly decreases the fault energy to save other systems from damage.

In the case of B & C MCBs the thermal trip never goes below 5s so you basically meet both 5s and 0.4s by exceeding the magnetic trip. Where as a D curve trip has two different current values one that trips in 5s, and the other where it has gone in to the instantaneous trip region.

Fuses are similar with two currents for two trip times, but the disconnect time decreases faster with overload. Hence for small currents they are less effective at overload protection than a MCB, but for very high fault currents (above the couple of kA region) the blow faster than the MCB magnetic trip and so limit the fault current & energy much better.

Except for rewirable BS3036 fuses, they are not good for more than 1-4kA anyway :(
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If you lack the on-site guide, then the simple "rule-of-thumb" is to check the worst-case fault current is greater than 6 times the OCPD rating. For example, in a 32A circuit you want 6 * 32 = 192A at 95% of 230V = 218V

Z = 218 / 192 = 1.14 ohms

OSG values are:
  • BS 3036 30A fuse 2.0 or 0.83 ohm (Table B1)
  • BS 88-2 32A fuse 1.4 or 0.79 ohm (Table B2)
  • 32A B breaker 1.1 ohm (Table B6)
  • 32A C breaker 0.55 ohm
  • 32A D breaker 0.55 or 0.28 ohm
So you can see the approximation is not eaxct, but it puts you in roughly the right range of values to ask "Is this sane?" when testing before you refer to the official values.
 
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BS7671 isn't that relevant here Max Zs is just Supply Voltatge/Trip Curve * Cmin - Ohms law V/I=R

i.e (230/Mcb 32A*5) * 0.95(rule of thumb if you want 0.80) =1.37 or 1.1 ohms etc

but OP question is interesting why is there a table for 0.4 and 5 secs for one but not the other?

The fusing factor I2 is 1.45 for MCB type B so 1 hour of overload at that current so where does 5 seconds come from?
 
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Look at the tripping curves, the B and C curve MCB's are already into their instantaneous tripping point at or before the 5s mark
exactly it's straight down from 10 seconds that tells us nothing ?
the only thing I can see is 5 sec is for distribution circuits of & above 32amp
 

pc1966

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Yep but what 5 second requirement I can't see one except 5 sec is for distribution circuits of & above 32amp ?
Those are your cases for a 5s disconnect time, subject to thermal limits on the cable (e.g. adiabatic check).

Normally you would always aim for 0.4s or below even if not explicitly required, but there are cases when you look at the 5s limit. For example:
  • For high current sub-main or fixed circuits you may not be able to meet 0.4s due to Ze anyway
  • On a sub-main you might push the upstream OCPD as high as the 5s or adiabatic thermal limits allow to get better selectivity with the down-stream DB MCBs.
Post automatically merged:

yeah both aren't really needed if you just do the simple equation.
Except when the simple equations is very wrong. Yes, if you know it is a B-curve MCB, but in other cases you can get it wrong by a factor of 2 or so.
 

pc1966

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Those are your cases for a 5s disconnect time, subject to thermal limits on the cable (e.g. adiabatic check).
The OSG book in Appendix B has tables that include the typical adiabatic limit for the common CPC sizes so it is your easiest option for selecting an OCPD when desiging something like a garage CU feed, or verifying an existing OCPD and measured Zs is reasonable.
 
I like the 35 way. I've always done it this way but its longer . b type 5 times, c type 10 times, d type 20 times. so if its a c type 32amp it would be 230 divided by 320=0.71. 0.71x 80%= 0.56. 0.56 x 0.95 c mim =0.55, long winded so in future might youse the magic 35.
 

pc1966

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What bothers me about "quick calculations" like 35/In is they end up being used and not understood. Yes, that works fine for a B-curve MCB but not for other cases!

So Farmelectrics method is better as it shows the key point - the 0.4s disconnection is achieved by ensuring the magnetic trip is reached under worst-case conditions (0.95 supply volts and 0.8 for cable temperature on R1+R2).
 
Can't say I'm a huge fan of this guy, seems a bit harsh after posting his video but I get the impression he doesn't know why it's 35 - also on his site he uses little r1 for a reading on a Radial and few other things ;) that a tutor should know better.
 

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