RCD classes clarification - Page 1
Forum section: electronics / general

author: jadew
author_rank: Frequent Contributor
timestamp: May 31, 2016, 08:30:42 pm
content: 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.

author: engineer_in_shorts
author_rank: Regular Contributor
timestamp: May 31, 2016, 09:47:07 pm
content: Leave alone if you are in the UK. No further protection from 10ma.

author: BradC
author_rank: Super Contributor
timestamp: June 01, 2016, 02:17:45 am
content: Quote from: jadew on May 31, 2016, 08:30:42 pmYeah. 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.

author: Brumby
author_rank: Unknown
timestamp: June 01, 2016, 04:07:55 am
content: This:Quote from: BradC on June 01, 2016, 02:17:45 am

author: jahonen
author_rank: Super Contributor
timestamp: June 01, 2016, 07:34:18 am
content: 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

author: jadew
author_rank: Frequent Contributor
timestamp: June 05, 2016, 01:23:48 pm
content: Thanks guys!Sorry for the late reply. Very good information.

author: karoru
author_rank: Regular Contributor
timestamp: June 22, 2016, 11:18:35 am
content: Quote from: BradC on June 01, 2016, 02:17:45 amMost 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.

author: BradC
author_rank: Super Contributor
timestamp: June 22, 2016, 11:17:16 pm
content: Quote from: karoru on June 22, 2016, 11:18:35 amThis is also why we don't have RCDs on fixed appliances.

author: karoru
author_rank: Regular Contributor
timestamp: June 23, 2016, 03:05:34 pm
content: 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.

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 23, 2016, 04:24:08 pm
content: 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.

author: BradC
author_rank: Super Contributor
timestamp: June 24, 2016, 09:15:38 am
content: Quote from: karoru on June 23, 2016, 03:05:34 pmYes 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.

author: Monkeh
author_rank: Super Contributor
timestamp: June 24, 2016, 12:05:38 pm
content: Quote from: Aodhan145 on June 23, 2016, 04:24:08 pmSadly that's not how discrimination works, and you'll likely end up having to reset both.

author: Towger
author_rank: Super Contributor
timestamp: June 24, 2016, 02:45:47 pm
content: Quote from: Monkeh on June 24, 2016, 12:05:38 pmI 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.

author: Monkeh
author_rank: Super Contributor
timestamp: June 24, 2016, 02:59:47 pm
content: Quote from: Towger on June 24, 2016, 02:45:47 pmExcept nearly any incident will cause enough leakage to trip BOTH. Or whichever is faster, which is pot luck.

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 24, 2016, 03:29:13 pm
content: Quote from: Monkeh on June 24, 2016, 02:59:47 pmI 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.

author: karoru
author_rank: Regular Contributor
timestamp: June 25, 2016, 01:32:38 am
content: Quote from: Monkeh on June 24, 2016, 02:59:47 pmIf 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:)