Type A rcd . EICR coding ? etc

True - I avoided the wider issues of already-installed RCDs in my earlier comment. This admittedly is the thread's main purpose: how to handle existing type-AC in inspection reports. But I think I should leave it to others, as I don't have much experience of these codes or their implications to installation owners. I do think that a lot of tolerance should be given to installations that are according to the rules of their day, unless there's good reason to suppose them dangerous due to statistics or to knowledge about new use-situations (such as the very different loads).  My main reason for posting was earlier comments about Europe and about whether preventing type AC for new installations is a bad idea. Now, thanks to interesting comments (Dave and Mike), I'm stimulated to give some description of general AC/A RCD features that might interest a few people here and be good background for your tests.


Yes, I agree that in the majority of cases the type AC would do the job, since earth faults (including accidental contact) would typically not have a diode present in the loop.  However, note that it's not just devices with a single-diode rectifier that could give a half-wave rectified residual current. A bridge rectifier can do this too, from either of its dc outputs. So half-wave rectified residual currents can arise from the wires and components that come after a rectifier inside an appliance, if these touch the frame or if a person directly contacts such parts. See the residual ('fault') component of current in cases 3,4,5,(&7) in the figure that Andy J posted earlier in this thread.


The RCD standards just specify tests that must be passed, without demanding how the device is to be implemented. However, the design of any moden, passive ('voltage independent') RCD of types A or AC that I've seen follows much the same pattern. An explanation of this design makes clear a lot about their behaviour and the polarity dependence, which was discussed in earlier comments. I'll try to give such an explanation below. The focus is on passive RCDs, known in the standards as 'voltage independent', meaning that they'll be able to trip even if there's no voltage between the conductors. Most RCCBs in Europe are of this type, although many recent RCBOs aren't.


-- Types A and AC


It's the trip mechanism that is polarity dependent. In order to trip with the very small power available from a small current-transformer, these mechanisms are not like normal relays or solenoids in which iron parts move in a magnetic circuit that includes air-gaps. Instead, they have a closed magnetic circuit, where a small permanent magnet holds the magnetic parts together against a spring. A coil of fine wire is wound around this circuit: a current through the coil in one direction will just add to the permanent magnet's field, but in the other direction it will counteract this field, and with enough current the spring will be able to separate the iron parts. I haven't seen any exception to this design in modern voltage-independent RCDs of types A or AC. The difference in trip-time when tripping on full-wave ac starting at 0 versus 180 degrees (affecting types A and AC, but typically with more delay for AC as can be inferred below) is a result of the polarity dependence of the tripping mechanism.


An RCD of type A needs a current transformer with a soft magnetic material. Consider a half-wave rectified residual current, i.e. pulses in one direction. In the good case, the first half of the half-wave pulse, rising from zero to peak residual current, induces a current in the trip coil in the right direction for tripping, so the RCD trips after whatever further delays the mechanical parts have. In the bad case, the first half of the half wave induces the 'wrong' (non-tripping) direction of current in the tripping coil. However, when the residual current returns to zero the magnetization of the soft material will also return largely to zero, inducing a current in the opposite direction in the tripping coil, so it still can trip.


The classic type AC has a much harder magnetic material in its current transformer. It's then mainly the first rising edge of a half-wave residual current that has a chance of causing a trip. That only happens if the residual current is in the right direction. After the first peak of residual current, the core will remain strongly magnetised until a residual current flows in the opposite direction. It doesn't help that the residual current just goes down to zero, as the magnetization remains. If the half-wave residual current builds up slowly, e.g. by moisture accumulation, the classic type-AC RCD would not trip in either polarity. However, as has been pointed out earlier in the thread, not all RCDs marked as AC nowadays have this classic behaviour - although some new ones I've tried in the last year still do.


Really old RCDs were big and heavy compared to modern ones (even passive modern ones). They used a big core in the current transformer in order to get enough power to operate the trip mechanism. Modern ones have not only a more sensitive tripping mechanism, but also very high-permeability current-transformers: relative permeabilities are tens of thousands. I don't know the history of these materials, but can imagine that some decades ago it might have been more difficult to obtain cores with the combination of such high permeability and magnetic softness, at reasonable price. Modern type A RCDs and their prices (when not seen as specialized devices) show us that it's now possible.


Regarding the size of magnetic cores: note that making a classic type-AC core bigger does very little to help make it more like type A. It's the material's nature that's important: it should be soft enough to give significant contribution to induced voltages during the return-to-zero of residual current. The 'pulsating-dc' issue, of AC versus A, is more about hysteresis than saturation. Saturation is relevant to steady dc preventing other signals being noticed - and there we move to thinking of type B, so let's stop...  It should also be noted that the input residual current is like a current source applied to the transformer, rather than a voltage source. Increasing an inductor's core area in this case doesn't much help to prevent saturation. (A further subtlety is the current in the secondary: with a varying residual current the induced secondary current can partly counteract the primary current, whereas steady dc gives no induced current after a while, in which case the entire primary residual current is used on magnetizing the core.)


Another feature of voltage-independent RCDs is that capacitors are often added in the trip-circuit, to increase the current through it (by resonance). This makes the device less sensitive to other frequencies than the intended one. The final common component is shunt diodes to limit the voltage applied to the tripping coil in the event of a large residual current.



-- Voltage dependent devices


Many small modern RCDs (typically the ones implemented in 1-module RCBOs) are 'voltage dependent'. They use the supply voltage to amplify the weak signal from a small current-transformer and to drive the tripping coil. I remember Dave described this a few months ago in a discussion of the power demand of such devices: a tripping coil can be in series with the electronics, across the 230V terminals. In these, the tripping coil can be a simple solenoid that pulls a magnetic 'slug', independent of polarity, as the power available to drive the coil is much more than in a voltage-independent RCD. I have seen one case where the electronic part was just a thyristor (not triac) supplied from the current transformer, so it anyway became polarity dependent, i.e. the tripping time varied between 0 & 180 degree settings of a tester). The use of a triac should allow a voltage-dependent RCD to be independent of polarity. That doesn't necessarily mean it's type A, as a gradually increased half-wave could still fail to trip in either direction if the core had a hard material. However, the demand on the core material's permeability and size is much less when its output is amplified, so it's easy to have a suitable material. Type A was already becoming common in voltage-dependent RCBOs in the UK in the past several years.


The traditional worry with a voltage-dependent RCD is if e.g. the neutral of a two-pole device is broken before the device. Then, with no voltage between conductors to drive the tripping circuit, it would be possible for large earth-fault currents to flow without a trip. In some regions the 'functional earth' connection has been used in order to have a chance to operate from a bit of current between a phase and this earth. That's not much use in a TN-C-S system unless the break is very close to the RCD, but it's helpful for other systems. TT is of course the one where the biggest danger exists from RCD non-operation. In other regions the functional earth is not popular, as the combination of neutral break and earth fault is considered too unlikely to warrant the hassle of an extra wire and troubles with insulation-testing.


Reliability of electronic components has been another criticism of the voltage-dependent devices, but the US and friends have been at it a long time with their 6 mA devices, and the reliability of very sensitive mechanical tripping mechanisms in voltage-independent RCDs leaves no strong case for the electronic choice being necessarily worse in reliability. I've found cute little baby spiders and web inside a tripping mechanism, stopping it from working.


Another issue of powered (voltage-dependent) devices lies behind Victoria's ban on some RCBO models that nevertheless fulfil the IEC requirements. The problem typically arises if supply and load sides are not connected as such, in which case some models can continue to power the tripping coil after tripping, resulting in its burning out so the device can't trip later. More about this prohibition here. I experienced this when first using the test-button of a via-ebay-imported 10 mA RCCB, before I read of the Australian experience. Mine had the clever safety feature of displaying flashing light and smoke to warn me that it probably wouldn't be on duty again.


--

True - I avoided the wider issues of already-installed RCDs in my earlier comment. This admittedly is the thread's main purpose: how to handle existing type-AC in inspection reports. But I think I should leave it to others, as I don't have much experience of these codes or their implications to installation owners. I do think that a lot of tolerance should be given to installations that are according to the rules of their day, unless there's good reason to suppose them dangerous due to statistics or to knowledge about new use-situations (such as the very different loads).  My main reason for posting was earlier comments about Europe and about whether preventing type AC for new installations is a bad idea. Now, thanks to interesting comments (Dave and Mike), I'm stimulated to give some description of general AC/A RCD features that might interest a few people here and be good background for your tests.


Yes, I agree that in the majority of cases the type AC would do the job, since earth faults (including accidental contact) would typically not have a diode present in the loop.  However, note that it's not just devices with a single-diode rectifier that could give a half-wave rectified residual current. A bridge rectifier can do this too, from either of its dc outputs. So half-wave rectified residual currents can arise from the wires and components that come after a rectifier inside an appliance, if these touch the frame or if a person directly contacts such parts. See the residual ('fault') component of current in cases 3,4,5,(&7) in the figure that Andy J posted earlier in this thread.


The RCD standards just specify tests that must be passed, without demanding how the device is to be implemented. However, the design of any moden, passive ('voltage independent') RCD of types A or AC that I've seen follows much the same pattern. An explanation of this design makes clear a lot about their behaviour and the polarity dependence, which was discussed in earlier comments. I'll try to give such an explanation below. The focus is on passive RCDs, known in the standards as 'voltage independent', meaning that they'll be able to trip even if there's no voltage between the conductors. Most RCCBs in Europe are of this type, although many recent RCBOs aren't.


-- Types A and AC


It's the trip mechanism that is polarity dependent. In order to trip with the very small power available from a small current-transformer, these mechanisms are not like normal relays or solenoids in which iron parts move in a magnetic circuit that includes air-gaps. Instead, they have a closed magnetic circuit, where a small permanent magnet holds the magnetic parts together against a spring. A coil of fine wire is wound around this circuit: a current through the coil in one direction will just add to the permanent magnet's field, but in the other direction it will counteract this field, and with enough current the spring will be able to separate the iron parts. I haven't seen any exception to this design in modern voltage-independent RCDs of types A or AC. The difference in trip-time when tripping on full-wave ac starting at 0 versus 180 degrees (affecting types A and AC, but typically with more delay for AC as can be inferred below) is a result of the polarity dependence of the tripping mechanism.


An RCD of type A needs a current transformer with a soft magnetic material. Consider a half-wave rectified residual current, i.e. pulses in one direction. In the good case, the first half of the half-wave pulse, rising from zero to peak residual current, induces a current in the trip coil in the right direction for tripping, so the RCD trips after whatever further delays the mechanical parts have. In the bad case, the first half of the half wave induces the 'wrong' (non-tripping) direction of current in the tripping coil. However, when the residual current returns to zero the magnetization of the soft material will also return largely to zero, inducing a current in the opposite direction in the tripping coil, so it still can trip.


The classic type AC has a much harder magnetic material in its current transformer. It's then mainly the first rising edge of a half-wave residual current that has a chance of causing a trip. That only happens if the residual current is in the right direction. After the first peak of residual current, the core will remain strongly magnetised until a residual current flows in the opposite direction. It doesn't help that the residual current just goes down to zero, as the magnetization remains. If the half-wave residual current builds up slowly, e.g. by moisture accumulation, the classic type-AC RCD would not trip in either polarity. However, as has been pointed out earlier in the thread, not all RCDs marked as AC nowadays have this classic behaviour - although some new ones I've tried in the last year still do.


Really old RCDs were big and heavy compared to modern ones (even passive modern ones). They used a big core in the current transformer in order to get enough power to operate the trip mechanism. Modern ones have not only a more sensitive tripping mechanism, but also very high-permeability current-transformers: relative permeabilities are tens of thousands. I don't know the history of these materials, but can imagine that some decades ago it might have been more difficult to obtain cores with the combination of such high permeability and magnetic softness, at reasonable price. Modern type A RCDs and their prices (when not seen as specialized devices) show us that it's now possible.


Regarding the size of magnetic cores: note that making a classic type-AC core bigger does very little to help make it more like type A. It's the material's nature that's important: it should be soft enough to give significant contribution to induced voltages during the return-to-zero of residual current. The 'pulsating-dc' issue, of AC versus A, is more about hysteresis than saturation. Saturation is relevant to steady dc preventing other signals being noticed - and there we move to thinking of type B, so let's stop...  It should also be noted that the input residual current is like a current source applied to the transformer, rather than a voltage source. Increasing an inductor's core area in this case doesn't much help to prevent saturation. (A further subtlety is the current in the secondary: with a varying residual current the induced secondary current can partly counteract the primary current, whereas steady dc gives no induced current after a while, in which case the entire primary residual current is used on magnetizing the core.)


Another feature of voltage-independent RCDs is that capacitors are often added in the trip-circuit, to increase the current through it (by resonance). This makes the device less sensitive to other frequencies than the intended one. The final common component is shunt diodes to limit the voltage applied to the tripping coil in the event of a large residual current.



-- Voltage dependent devices


Many small modern RCDs (typically the ones implemented in 1-module RCBOs) are 'voltage dependent'. They use the supply voltage to amplify the weak signal from a small current-transformer and to drive the tripping coil. I remember Dave described this a few months ago in a discussion of the power demand of such devices: a tripping coil can be in series with the electronics, across the 230V terminals. In these, the tripping coil can be a simple solenoid that pulls a magnetic 'slug', independent of polarity, as the power available to drive the coil is much more than in a voltage-independent RCD. I have seen one case where the electronic part was just a thyristor (not triac) supplied from the current transformer, so it anyway became polarity dependent, i.e. the tripping time varied between 0 & 180 degree settings of a tester). The use of a triac should allow a voltage-dependent RCD to be independent of polarity. That doesn't necessarily mean it's type A, as a gradually increased half-wave could still fail to trip in either direction if the core had a hard material. However, the demand on the core material's permeability and size is much less when its output is amplified, so it's easy to have a suitable material. Type A was already becoming common in voltage-dependent RCBOs in the UK in the past several years.


The traditional worry with a voltage-dependent RCD is if e.g. the neutral of a two-pole device is broken before the device. Then, with no voltage between conductors to drive the tripping circuit, it would be possible for large earth-fault currents to flow without a trip. In some regions the 'functional earth' connection has been used in order to have a chance to operate from a bit of current between a phase and this earth. That's not much use in a TN-C-S system unless the break is very close to the RCD, but it's helpful for other systems. TT is of course the one where the biggest danger exists from RCD non-operation. In other regions the functional earth is not popular, as the combination of neutral break and earth fault is considered too unlikely to warrant the hassle of an extra wire and troubles with insulation-testing.


Reliability of electronic components has been another criticism of the voltage-dependent devices, but the US and friends have been at it a long time with their 6 mA devices, and the reliability of very sensitive mechanical tripping mechanisms in voltage-independent RCDs leaves no strong case for the electronic choice being necessarily worse in reliability. I've found cute little baby spiders and web inside a tripping mechanism, stopping it from working.


Another issue of powered (voltage-dependent) devices lies behind Victoria's ban on some RCBO models that nevertheless fulfil the IEC requirements. The problem typically arises if supply and load sides are not connected as such, in which case some models can continue to power the tripping coil after tripping, resulting in its burning out so the device can't trip later. More about this prohibition here. I experienced this when first using the test-button of a via-ebay-imported 10 mA RCCB, before I read of the Australian experience. Mine had the clever safety feature of displaying flashing light and smoke to warn me that it probably wouldn't be on duty again.


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