Merry Christmas everyone.
1.5.5.3 stated that subcircuits supplying fixed equipment, for example a wall oven has a maximum disconnection time of 5 seconds.
In most residential situations, a wall oven and other fixed equipment will be protected by a Type C MCB and no RCD.
In regards to testing, 8.3.5.2, states that the resistance of the PEC must be low enough to allow enough current to operate the protective device within the required time. It also refers to table 8.2 in the notes.
When looking at tables 8.1 and 8.2 the values under type C MCB seem to be for a 0.4s disconnection time (stated it table 8.1 but not 8.2). There is also a note in 8.1 that says a 5s disconnection time for circuit breakers is not shown as they are intended to operate in the instantaneous tripping zone. I'm not too sure that this means. Only fuses are shown as having the 5s disconnection time.
My question is that based on section 8, does it mean that all subcircuits protected by an MCB have to trip withing 0.4s regardless of whether it is supplying fixed appliances, sockets, lighting etc or not?
Protective device disconnection times
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ElectricalApprentice
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Protective device disconnection times
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- Mazdaman (Sun Dec 27, 2020 9:10 pm)
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Re: Protective device disconnection times
No; it doesn't mean that at all.
The max acceptable disconnection times are set differently due to different levels of risk;
as in there's much less risk of shock from fixed equipment than from portable equipment. This relates to providing "fault protection"; ie protection against shock from contact with conductive parts not normally live, but live under (earth) fault conditions. Used to be called 'indirect contact'; as against 'direct contact' with parts that are normally alive - which is dealt with by providing "basic protection".
To start getting your head around this stuff, you have to read right through 1.5.3 protection against electric shock; then 1.5.4 which gives details relating to basic protection, 1.5.5 which gives details relating to "fault protection", and 1.5.6 which gives details relating to additional protection.
You also need to read 2.4 "fault protection".
Fault protection is about ensuring that the voltage on earthed parts never rises to a hazardous level due to earth fault currents.
Importantly there are several permitted methods of providing "fault protection"; the most common being automatic disconnection of supply [1.5.5.3]; which both PEC testing [8.3.5] & EFLI testing [8.3.9] relate to.
The reason Tables 8.1 & 8.2don't include data for a 5 sec disconnection time for mcbs is that it wouldn't seve any useful purpose.
All permitted Types of mcbs have a magnetic trip for high-current type faults (eg short circuit faults & earth faults); in addition to the thermally operated tripping for overload faults.
The magnetic trip is almost instantaneous, and doesn't vary significantly depending on actual current level. As soon as the current reaches the threshold level; the device will operate.
If you look up the operating curves for various Types of mcb; you'll find a section of the curve that's almost vertical, ie the operating time doesn't significantly change over that range of currents; and another part that's a sweeping curve where the lower the current the longer it takes for the device to operate. That vertical part of the curve is the "instantaneous tripping zone", where operation is magnetic rather than thermal.
Section 8 doesn't set the requirements for fault protection. The requirements are set in 1.5.5 , and also in 5.7.
All section 8 does is require us to test that we've actually met the requirement in practice, as against just designing in theory and assuming it's all been installed correctly.
Just as the max permitted operating time varies based on risk of someone touching parts that are live, at a hazardous level ,due to an earth fault conditions; the testing requirements are based on that risk.
First we are required to test the PEC part of the earth fault loop in EVERY case; regardless of the trip time requirements . That's individually test that every item required to be earthed actually is earthed back to switchboard it's supplied from (applying Re values from table 8.2). That's considered enough for most circuits.
But for circuits supplying socket outlets, because there' a higher risk of someone touching equipment that's plugged in, we have to go further and test the the earth fault loop impedance (EFLI). Testing the entire EFL back to source at transformer (applying Table 8.1) is preferable; and gives the most accurate results . But that requires supply to be available, and sometimes it's not. If it's not; then we can use the alternative method of just testing parts of the EFL that are in the final subcircuit ; ie test A+E (applying Rphe values from Table 8.2); and we're allowed to assume that the rest of the EFL - in the distribution system, mains, and submains - is OK.
Basically the Tables give us benchmarks - for each test method that allow us to say either "yes, it will operate within max permitted time", or "no, it might not operate within max permitted time".
There's NO chance of any mcb not operating much faster than 5 sec; so there's no point calculating & publishing values of Re, or Z, that are deemed to comply. If we're using an mcb as the means of protection; then under earth fault conditions the device will always operate well under 5 seconds. But it may not operate under 0.4 seconds, so we need the deemed-to-comply values in the Tables in order to prove that the earth fault loop is sufficiently low impedance. The alternative to looking up the Tables would be doing a separate calculation for each circuit to check whether our measured value is OK.
Whereas a (HRC) fuse is purely thermal; so we need deemed-to-comply values to establish compliance even for a circuit with 5 sec max trip time.
It would occasionally be useful to have values for HRCs operating within 0.4 sec; but how often these days do we actually install subcircuits supplying sockets that are protected by fuses? If we do; we can do the calculation - clause 5.7.4 provides the formula, the maths isn't hard, and Appendix B provides a full explanation. And for anything else; all we need is the values for 5 sec.
Note this formula is different from the one in clause 2.5.4.5 for short circuit protection. This is "fault protection; which is a different tghing entirely - the only bit that's the same is that we generally use the same overcurrent device to do the disconnection for ip to 3 types of protection: fault protection, short curcuit protection, and overload protection. The big difference here is that fault protection is about electric shock; while the others both relate to protection from thermal damage & fire.
Even for sockets; we don't always have to test EFLI;
Firstly we may not be using 'automatic disconnection of supply" as the means of providing fault protection. For the other methods; which don't rely on earthing, the EFLI value doesn't matter (but there may be other testing required, eg the equipotential bonding network for circuits supplied from a separated supply).
Secondly, we may be using an RCD instead of an over-current device to do the disconnecting; as permitted by 1.5.5.3 & 2.4.3.
Like an mcb, an RCD will operate very quickly. The max permitted operating time for the approved devices, at rated operating current, is 300 ms (0.3 s; ie 25 % faster than we need for fault protection of circuits supplying sockets).
But unlike an mcb, or a fuse; it doesn't require a high fault current in order to operate within the time. It will operate at very low current. Most of the RCDs we install, have a rated operating current no greater than 30 mA. A simple application of Ohm's Law shows that in 230 V supply the EFLI can be up to 7666.66 ohms and it will still work. Even a 300 mA RCD will still operate within time as long as the impedance is below 766.6 ohms; however care is needed with Type s RCDs that have a bult-in delay.
So, barring what amounts to nearly an open-circuit PEC; there's NO chance of an RCD (other than Type S) taking too long to operate -and we've already tested the PEC part and shown it to be no more than Table 8.2 allows for the rating of the overcurrent device; so we know the total EFLI can't be much more than twice that. Since the actual precise value of EFLI simply doesn't matter; there's no point testing it.
--------------------------
All this is based on assessment of the risk of someone getting a shock. The same assessment is applied to "additional protection" by RCD; as required under clause 2.6. For this clause; the risk is considered based on the nature and / or location of the equipment.
Currently, for NZ domestic & residential, subcircuits supplying either equipment that plugs in; or directly connected hand held equipment, or lighting; are treated as presenting a greater risk than other directly connected equipment.
For other types of installation, again risk is the deciding factor and a look at the 2018 edition includes a range of equipment types and locations that are deemed to present high enough risk that RCD protection is required.
This "additional protection" is "additional" to the basic protection already provided by one of the methods permitted by 1.5.4. RCDs are not recognised as providing basic protection by themselves. But they are recognised, by clause 2.4.3, as providing fault protection by themselves.
------------------
Worth noting that most, if not all, Arc fault detection devices available in NZ include a RCD function as well. So an AFDD that has a 100 mA or 300 mA RCD built in can provide "fault protection"; but not "additional protection".
The max acceptable disconnection times are set differently due to different levels of risk;
as in there's much less risk of shock from fixed equipment than from portable equipment. This relates to providing "fault protection"; ie protection against shock from contact with conductive parts not normally live, but live under (earth) fault conditions. Used to be called 'indirect contact'; as against 'direct contact' with parts that are normally alive - which is dealt with by providing "basic protection".
To start getting your head around this stuff, you have to read right through 1.5.3 protection against electric shock; then 1.5.4 which gives details relating to basic protection, 1.5.5 which gives details relating to "fault protection", and 1.5.6 which gives details relating to additional protection.
You also need to read 2.4 "fault protection".
Fault protection is about ensuring that the voltage on earthed parts never rises to a hazardous level due to earth fault currents.
Importantly there are several permitted methods of providing "fault protection"; the most common being automatic disconnection of supply [1.5.5.3]; which both PEC testing [8.3.5] & EFLI testing [8.3.9] relate to.
The reason Tables 8.1 & 8.2don't include data for a 5 sec disconnection time for mcbs is that it wouldn't seve any useful purpose.
All permitted Types of mcbs have a magnetic trip for high-current type faults (eg short circuit faults & earth faults); in addition to the thermally operated tripping for overload faults.
The magnetic trip is almost instantaneous, and doesn't vary significantly depending on actual current level. As soon as the current reaches the threshold level; the device will operate.
If you look up the operating curves for various Types of mcb; you'll find a section of the curve that's almost vertical, ie the operating time doesn't significantly change over that range of currents; and another part that's a sweeping curve where the lower the current the longer it takes for the device to operate. That vertical part of the curve is the "instantaneous tripping zone", where operation is magnetic rather than thermal.
Section 8 doesn't set the requirements for fault protection. The requirements are set in 1.5.5 , and also in 5.7.
All section 8 does is require us to test that we've actually met the requirement in practice, as against just designing in theory and assuming it's all been installed correctly.
Just as the max permitted operating time varies based on risk of someone touching parts that are live, at a hazardous level ,due to an earth fault conditions; the testing requirements are based on that risk.
First we are required to test the PEC part of the earth fault loop in EVERY case; regardless of the trip time requirements . That's individually test that every item required to be earthed actually is earthed back to switchboard it's supplied from (applying Re values from table 8.2). That's considered enough for most circuits.
But for circuits supplying socket outlets, because there' a higher risk of someone touching equipment that's plugged in, we have to go further and test the the earth fault loop impedance (EFLI). Testing the entire EFL back to source at transformer (applying Table 8.1) is preferable; and gives the most accurate results . But that requires supply to be available, and sometimes it's not. If it's not; then we can use the alternative method of just testing parts of the EFL that are in the final subcircuit ; ie test A+E (applying Rphe values from Table 8.2); and we're allowed to assume that the rest of the EFL - in the distribution system, mains, and submains - is OK.
Basically the Tables give us benchmarks - for each test method that allow us to say either "yes, it will operate within max permitted time", or "no, it might not operate within max permitted time".
There's NO chance of any mcb not operating much faster than 5 sec; so there's no point calculating & publishing values of Re, or Z, that are deemed to comply. If we're using an mcb as the means of protection; then under earth fault conditions the device will always operate well under 5 seconds. But it may not operate under 0.4 seconds, so we need the deemed-to-comply values in the Tables in order to prove that the earth fault loop is sufficiently low impedance. The alternative to looking up the Tables would be doing a separate calculation for each circuit to check whether our measured value is OK.
Whereas a (HRC) fuse is purely thermal; so we need deemed-to-comply values to establish compliance even for a circuit with 5 sec max trip time.
It would occasionally be useful to have values for HRCs operating within 0.4 sec; but how often these days do we actually install subcircuits supplying sockets that are protected by fuses? If we do; we can do the calculation - clause 5.7.4 provides the formula, the maths isn't hard, and Appendix B provides a full explanation. And for anything else; all we need is the values for 5 sec.
Note this formula is different from the one in clause 2.5.4.5 for short circuit protection. This is "fault protection; which is a different tghing entirely - the only bit that's the same is that we generally use the same overcurrent device to do the disconnection for ip to 3 types of protection: fault protection, short curcuit protection, and overload protection. The big difference here is that fault protection is about electric shock; while the others both relate to protection from thermal damage & fire.
Even for sockets; we don't always have to test EFLI;
Firstly we may not be using 'automatic disconnection of supply" as the means of providing fault protection. For the other methods; which don't rely on earthing, the EFLI value doesn't matter (but there may be other testing required, eg the equipotential bonding network for circuits supplied from a separated supply).
Secondly, we may be using an RCD instead of an over-current device to do the disconnecting; as permitted by 1.5.5.3 & 2.4.3.
Like an mcb, an RCD will operate very quickly. The max permitted operating time for the approved devices, at rated operating current, is 300 ms (0.3 s; ie 25 % faster than we need for fault protection of circuits supplying sockets).
But unlike an mcb, or a fuse; it doesn't require a high fault current in order to operate within the time. It will operate at very low current. Most of the RCDs we install, have a rated operating current no greater than 30 mA. A simple application of Ohm's Law shows that in 230 V supply the EFLI can be up to 7666.66 ohms and it will still work. Even a 300 mA RCD will still operate within time as long as the impedance is below 766.6 ohms; however care is needed with Type s RCDs that have a bult-in delay.
So, barring what amounts to nearly an open-circuit PEC; there's NO chance of an RCD (other than Type S) taking too long to operate -and we've already tested the PEC part and shown it to be no more than Table 8.2 allows for the rating of the overcurrent device; so we know the total EFLI can't be much more than twice that. Since the actual precise value of EFLI simply doesn't matter; there's no point testing it.
--------------------------
All this is based on assessment of the risk of someone getting a shock. The same assessment is applied to "additional protection" by RCD; as required under clause 2.6. For this clause; the risk is considered based on the nature and / or location of the equipment.
Currently, for NZ domestic & residential, subcircuits supplying either equipment that plugs in; or directly connected hand held equipment, or lighting; are treated as presenting a greater risk than other directly connected equipment.
For other types of installation, again risk is the deciding factor and a look at the 2018 edition includes a range of equipment types and locations that are deemed to present high enough risk that RCD protection is required.
This "additional protection" is "additional" to the basic protection already provided by one of the methods permitted by 1.5.4. RCDs are not recognised as providing basic protection by themselves. But they are recognised, by clause 2.4.3, as providing fault protection by themselves.
------------------
Worth noting that most, if not all, Arc fault detection devices available in NZ include a RCD function as well. So an AFDD that has a 100 mA or 300 mA RCD built in can provide "fault protection"; but not "additional protection".
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