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.