RCD classes clarification

[jadew]

Hey guys,

I'm planning to change my circuit breakers and install 10 mA RCDs on each circuit and maybe a 30 mA main one, for extra safety, in case one of the 10 mA fails to trigger.

The problem is that there are multiple classes of RCDs and I'm not sure I understand what the difference is. Apparently some are more suited for industrial use.

The classes I came across are AC, A and B. Can someone please shed some light on this?

Thanks.

[engineer_in_shorts]

Leave alone if you are in the UK. No further protection from 10ma.

[BradC]

Hey guys,

I'm planning to change my circuit breakers and install 10 mA RCDs on each circuit and maybe a 30 mA main one, for extra safety, in case one of the 10 mA fails to trigger.

Yeah. Don't do that.
If people could reliably use 10mA RCD's in houses they'd all have 10mA RCD's.
You will very likely find various appliances triggering the 10mA devices at the worst possible time. 30mA was settled on because it was less likely to get tripped in a household environment yet still provide sufficient protection against accident or stupidity.
10mA in a hospital environment is another kettle of fish however.

I have several devices here with mis-designed common-mode input filtering that will trip a 10mA RCD under the right circumstances, and I've seen motorized devices (like fridge compressors) do it too.

[Brumby]

This:

You will very likely find various appliances triggering the 10mA devices at the worst possible time. 30mA was settled on because it was less likely to get tripped in a household environment yet still provide sufficient protection against accident or stupidity.

[jahonen]

AC type RCD works (trips) only with sinusoidal residual current, A type works with either sinusoidal or pulsed (rectified) current, and B type works with sinusoidal, pulsed or DC current.

Finnish standard (based on HD 60364) for electrical installations states that AC type should not be used in new installations, so RCD must be of type A or B.

Cascading 30 and 10 mA RCDs may cause problems with selectivity, so higher rated RCD may trip before smaller one.

Regards,
Janne

[jadew]

Thanks guys!

Sorry for the late reply. Very good information.

[karoru]

I have several devices here with mis-designed common-mode input filtering that will trip a 10mA RCD under the right circumstances, and I've seen motorized devices (like fridge compressors) do it too.

Most old heating elements (like in ovens) will happily leak 10mA to earth when they become hot. I've had many pointless discussions with appliance repair technicians (who by some reason never have RCDs or proper earth connection at their workshops) that heaters found on the dumpster they put during repair tend to be not OKand testing insulation with 5$ DMM on ohms and "it heats up when connected to mains" is not a proper methodology to assess if a heating element is ok.

[BradC]

Most old heating elements (like in ovens) will happily leak 10mA to earth when they become hot.

This is also why we don't have RCDs on fixed appliances.

[karoru]

Kitchen stove? Of course. Devices like combi microwave-grills are usually connected to normal wall socket and for example in Poland it's mandatory to use RCD on wall-socket circuits.

[Aodhan145]

I was putting a 10mA one on my bench to trigger before the houses. My family does not appreciate all of the electricity going out.

[BradC]

Kitchen stove? Of course. Devices like combi microwave-grills are usually connected to normal wall socket and for example in Poland it's mandatory to use RCD on wall-socket circuits.

Yes of course. It has been so long since I'd seen one of those I'd completely forgotten they exist. I cooked with one every night while I worked in the middle east. They had 30mA RCD's there also.

[Monkeh]

I was putting a 10mA one on my bench to trigger before the houses. My family does not appreciate all of the electricity going out.

Sadly that's not how discrimination works, and you'll likely end up having to reset both.

[Towger]

I was putting a 10mA one on my bench to trigger before the houses. My family does not appreciate all of the electricity going out.

Sadly that's not how discrimination works, and you'll likely end up having to reset both.

I assume Aidan means it is placed after the house's sockets RCD, but should trip before it. There is a good change both will trip on anything more than low leakage, but should give some additional protection to him.

[Monkeh]

I was putting a 10mA one on my bench to trigger before the houses. My family does not appreciate all of the electricity going out.

Sadly that's not how discrimination works, and you'll likely end up having to reset both.

I assume Aidan means it is placed after the house's sockets RCD, but should trip before it. There is a good change both will trip on anything more than low leakage, but should give some additional protection to him.

Except nearly any incident will cause enough leakage to trip BOTH. Or whichever is faster, which is pot luck.

[Aodhan145]

I was putting a 10mA one on my bench to trigger before the houses. My family does not appreciate all of the electricity going out.

Sadly that's not how discrimination works, and you'll likely end up having to reset both.

I assume Aidan means it is placed after the house's sockets RCD, but should trip before it. There is a good change both will trip on anything more than low leakage, but should give some additional protection to him.

Except nearly any incident will cause enough leakage to trip BOTH. Or whichever is faster, which is pot luck.

I meant it both ways, I have had one or two close encounters but never a shock. I thought it might also trigger before the houses and prevent it from tripping, if it doesn't its not that big of a deal.

[karoru]

Except nearly any incident will cause enough leakage to trip BOTH. Or whichever is faster, which is pot luck.

If earthing is good enough, you just connected your live wire to earth and you're near enough to point of neutral wire - earth connection on power grid it will trip both RCDs and one's main circuit breakers.

And it's a pot luck, RCDs parameters are "will trip after <= miliseconds after | current_on_lives - current_on_neutral | > xx mA", conclusion that smaller RCD will trip first is just wishful thinking:)