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My war against dual rcd boards

Newfutile
As each (RCD) Residual Current Device must not have more than 30% leakage current on it.
It's hard to see how dual (RCD) Residual Current Device boards can be fitted at all these days.
i come across so many dual rcd boards with solar, evse and heat pumps on them, these have all been recently installed.
i wonder if BS7671 should state: Dual (RCD) Residual Current Device boards shall not be fitted, unless it can be shown (and documented) that they are suitable for the combined leakage currents expected .
otherwise developers will keep specifying them and i will have keep educating them on the many reasons a type A rcd should not be shared with other equipment!.
maybe manuafactuers instructions should also state : not suitable for a shared rcd, for certain equipment.
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  • The question that Graham asked was quite specific:

    ”However, if the lamp is damaged, and the user is being protected against accidental contact with live parts, say after the rectifier, would the type AC RCD operate is perhaps another?”

    in that he said “after the rectifier”, so assume there are some diodes charging a capacitor, which discharges to supply the DC current, and the input current is 25 mA at 230 volts, what is the DC output current and voltage?

  • in reply to Sparkingchip

    You tell me. 

    The input is 6.44 W, so the product of the output current and voltage must be no more than that.

    If there is a DC fault, there is no reason to believe that a Type AC RCD would trip, but it might. Graham will correct me if I have misunderstood.

    LED car headlamps glow with a supply of nominally 12 V, so that would give potentially 0.5 A @ 12 V. The figure that I have in my memory for the resistance of a human body finger-to-toe is 2 kΩ, so now we are looking at a current of 12/2000 = 6 mA.

  • in reply to Chris Pearson

    The figure that I have in my memory for the resistance of a human body finger-to-toe is 2 kΩ, so now we are looking at a current of 12/2000 = 6 mA.

    Indeed such a figure is quite credible- and  a glancing dry contact with mains is not normally fatal - just painful. But, not always ! and certainly there are enough fatalities to justify the desire to take steps to protect against mains shocks.

    That resistance falls if the shock current is sustained, as the skin chars at the point of contact, and the current then rises - the core of the body is much lower resistance, and damaged or wet skin removes most of that high resistance, allowing a far more dangerous current to flow.

    Also be aware that a lot of the small LED lights use a series capacitor dropper to set the LED current, and the mains current is higher than the wattage may suggest, as the power factor is poor. If the LEDs total 100V and a the rest is capacities, then the current is more than double the wattage divided by 230.

    Mike.

  • in reply to Chris Pearson
    Chris Pearson said:
    You tell me

    I asked AI to explain it, because it's a long time since I did this at college! The DC voltage will be around 330 volts:

    "In a simple diode-capacitor circuit (a half-wave rectifier with a smoothing capacitor), the capacitor charges to nearly the peak voltage (Vpeakcap V sub p e a k end-sub) of the AC input, which is about 1.414 times the input RMS voltage (Vrmscap V sub r m s end-sub), minus a small diode voltage drop (Vdcap V sub d). So, Vpeak≈(Vrms×1.414)−Vdcap V sub p e a k end-sub is approximately equal to open paren cap V sub r m s end-sub cross 1.414 close paren minus cap V sub d≈(×1.414)−, resulting in a DC output close to the peak value, not the RMS value, with some ripple.”.

  • in reply to mapj1
    mapj1 said:
    Also be aware that a lot of the small LED lights use a series capacitor dropper to set the LED current, and the mains current is higher than the wattage may suggest, as the power factor is poor. If the LEDs total 100V and a the rest is capacities, then the current is more than double the wattage divided by 230.

    Yes, but Sparkingchip specified the input current and voltage, so in this example, the wattage of the LED itself is less than 6.44 W, say 3 W.

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  • in reply to mapj1
    mapj1 said:
    Also be aware that a lot of the small LED lights use a series capacitor dropper to set the LED current, and the mains current is higher than the wattage may suggest, as the power factor is poor. If the LEDs total 100V and a the rest is capacities, then the current is more than double the wattage divided by 230.

    Yes, but Sparkingchip specified the input current and voltage, so in this example, the wattage of the LED itself is less than 6.44 W, say 3 W.

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