Does it make sense to add an additional RCD to your electronics workbench? - Page 1
Forum section: electronics / general

author: Ranayna
author_rank: Super Contributor
timestamp: March 01, 2019, 10:21:32 am
content: Hi everyone,I am in the process of planning a new electronics workbench combined with a dual-monitor PC workspace.My current planning phase is how I will setup the power distribution. Where the bench will be placed, there are 2 separate outlets available.Currently, I have a switchable powerstrip connected to each of them, one where the PC and it's peripherals are connected, the other one has my test equipment and workbench lighting. Room lighting is complexly separate.The building, as is typical for residences in Germany, has one central RCD covering all circuits. That RCD triggers at 30mA.Now I'm considering if it might be sensible to add a secondary 10mA RDC to the powerstrip for my test equipment. Something like this:https://www.en.bachmann24.com/bachmann-step-alu-1x-fi-10ma-mit-gst18-906-067.html?___store=en34&store=&___from_store=de34Does that even make sense, or would this be a false sense of additional security? Might this create issues with nuisance tripping because 10mA is too low?

author: Specmaster
author_rank: Super Contributor
timestamp: March 01, 2019, 01:00:58 pm
content: Quote from: Ranayna on March 01, 2019, 10:21:32 amI don't think it would be any good as many items may intentionally have a slight leakage to ground as filters etc.

author: rhb
author_rank: Super Contributor
timestamp: March 01, 2019, 10:44:03 pm
content: The March 2019 issue of QST has an article suggesting adding what is called a Ground Fault Circuit Interrupter in the US. I may do that, though I'm really more interested in an isolation transformer.I already have a much more elaborate power distribution system with separate circuits for T&M, lights and soldering gear.10 mA is a lot more current than several crappy SMPSUs will bleed to ground, so if the unit you are considering tripped, I'd be looking for the fault until I found it. And be very glad I had it.

author: Specmaster
author_rank: Super Contributor
timestamp: March 02, 2019, 01:03:31 am
content: Quote from: rhb on March 01, 2019, 10:44:03 pmA Ground Fault Circuit Interrupter is the same animal as an RCD (https://en.wikipedia.org/wiki/Residual-current_device) and works in precisely the same way, but has a different name thats all. The 10mA is completely misleading, a good RCD or GFCI will start tripping at around 3 to 5mA leakage to ground and in many cases there will be no discrimination and local device you install to protect your work bench and the other device in the distribution board and can frequently trip out both devices.

author: magic
author_rank: Super Contributor
timestamp: March 02, 2019, 07:40:36 am
content: It's a good idea in the sense that if something you are working on trips the RCD, it won't reboot your computer, reset the microwave's clock and so on.The same reason why power strips with built-in fuses are nice.

author: Specmaster
author_rank: Super Contributor
timestamp: March 02, 2019, 09:15:25 am
content: Quote from: magic on March 02, 2019, 07:40:36 amNo, unless the amount of leakage is enough to only trip the 10mA unit. In practise it is very likely that it would also take out the main 30mA unit as well. A 30mA must trip instantanously at 30mA if it goes at 31mA it is deemed a failure and dangerous and would be rejected at test in the factory. As units age, their tolerances drift and so manufacturers will calibrate their tripping curves to blow at a much lower figure typically 22mA or lower to accommodate ageing and reduce the reject rate.Another thing you need to be aware of is that some RCDs are capable of being desensitised by any DC leakage and may be completely disabled if sufficient DC is present to saturate the torriodal core of sensing coil. If this happens then it becomes a switch only.

author: Shock
author_rank: Super Contributor
timestamp: March 02, 2019, 09:40:49 am
content: Quote from: Specmaster on March 02, 2019, 09:15:25 amGot a source to any discussions or whitepapers on that? Sounds interesting reading.

author: Ian.M
author_rank: Super Contributor
timestamp: March 02, 2019, 09:44:29 am
content: For an electronics bench, where hazardous voltages may be present after mains is removed, and hot and sharp objects and tools are likely to be in use, and chemical hazards may be present, it is IMHO a serious safety hazard if tripping the RCD/GFCI protection also trips both the room and bench lighting circuits. I would recommend at least one LED emergency lighting fitting above the bench so there is always enough light to safely extricate yourself from hazards if you trip the building RCD/GFCI.An additional layer of RCD/GFCI protection for the PC and electronics test equipment offers little benefit. However, local RCD/GFCI protection that wont trip the whole building's RCD/GFCI, is extremely valuable if you are ever going to work on the PSUs or other non-isolated circuits of mains powered devices.If you take an isolating transformer and bond secondary side neutral to ground, (which defeats its isolation function), it will prevent any upstream RCD/GFCIs tripping, but a RCD/GFCI downstream of the secondary neutral to ground bond will trip on any secondary side fault.An 1200VA isolating transformer set up with removable links to bond secondary neutral to true ground, and to the output socket ground terminal, with a 6A/10mA RCBO (the lowest rating commonly available) is very useful - use it with links in place when you are working on devices with live mains circuits, and the local RCBO will trip instead of the whole building RCD/GFCI.With the links removed the RCBO is down-rated to an ordinary circuit breaker and there is no RCD protection, but the secondary side is then fully floating (until you connect grounded test equipment), with the transformer providing isolation.I wont go on about the relative benefits and hazards of RCD/GFCIs vs isolation transformers without any secondary side ground connection, as that's been done to exhaustion in many EEVblog forum threads over the years, except to note that RCD/GFCI protection is preferable for users with less experience and training.Quote from: Specmaster on March 02, 2019, 09:15:25 am*MANY* *THANKS* for drawing this to our attention. *MOST* modern RCD/GFCIs should trip on pulsating DC (IIRC its a requirement in most countries), but if you are buying them for electronics workbench use, check the datasheet!

author: Ice-Tea
author_rank: Super Contributor
timestamp: March 02, 2019, 09:53:52 am
content: Sure the main RCD triggers at 30mA? Over here, the standard is 300mA, supplemented with a 30mA for "wet" rooms. On my bench at the office I added a 10mA RCD (mind you: they are quite expensive compared...).

author: Specmaster
author_rank: Super Contributor
timestamp: March 02, 2019, 09:56:52 am
content: Quote from: Shock on March 02, 2019, 09:40:49 amNone that I can lay my hands on at the moment but I used to work for a major manufacturer and it is well known that DC will cease all transforming in a transformer. Take a look at the circuit of a RCD and it all revolves around a transformer designed to pick up extremely small differences between power going and out of the device so it only needs a very small amount DC being present to have an effect on the detection. Maybe if you do a Google search you might find the documentation on it?

author: Shock
author_rank: Super Contributor
timestamp: March 02, 2019, 10:37:17 am
content: Quote from: Specmaster on March 02, 2019, 09:56:52 amThanks I'll start with these two. Type B seems to be DC sensitive.http://www.doepke.co.uk/download/Techpub-17https://www.schneider-electric.com/resources/sites/SCHNEIDER_ELECTRIC/content/live/FAQS/328000/FA328983/en_US/earth%20leakage%20protection%20selection.pdf

author: Specmaster
author_rank: Super Contributor
timestamp: March 02, 2019, 11:44:47 am
content: Quote from: Shock on March 02, 2019, 10:37:17 amThat's the point, type "B" should be selected but often in reality they aren't due to the cost implications on the average installation where DC is considered not to be a high risk factor. If however, you do decide to try and implement a RCD nearer to the bench and therefore downstream of the device in the distribution board, then that device should be one is going to be DC sensitive to prevent the entire installation being compromised. As we are involved with electronics and as we know, therefore DC is always going to be present in some form or other so we should be aware of the potential of that reaching the RCD and its effect on its ability to perform its function.I first came across with a particular train station where the staff reported that from time to time that customers would report the occasional tingle being felt when they used certain apparatus in the washroom. The circuit was protected by a correctly rated RCD (30mA) and when they tested the device with a RCD tester, it tripped 99% of the time unless there was a train either in the station or very close by. I knew that trains of that era used a lot DC controls to control the motors and also signalling I was given to understand so I arranged for the existing RCD (made by another competing company) to be replaced with one that my company produced that I knew was not affected by DC. This was fitted and the problem was immediately resolved.Another example of this having an effect was presented to me an electrical consulting engineer who was a keen gardener and he used to take a portable colour TV to his garden shed for his grandchild to watch while she visited and he was working in his garden. The TV was plugged into a RCD socket and he noted that open testing the socket using the builtin test button it failed to trip when the TV was connected. Unplug the TV and it would trip every time, I gave him one of my companies RCD sockets as test and it worked perfectly and that particular consulting engineer switched over all of his work to my companies products on all subsequent projects he did and so did most of the consultants in that practise.So the problem is a real one, it might be only the odd isolated fault that but that one fault could perhaps be the difference maker if a person then subsequently touches a live connection somewhere else on the installation that should have been disconnected under normal conditions if the DC was not there.

author: Specmaster
author_rank: Super Contributor
timestamp: March 02, 2019, 11:53:42 am
content: Quote from: Ian.M on March 02, 2019, 09:44:29 amYes, most RCD's will still trip on pulsating DC as it just that pulsating and is more than likely to be at worst, twice the mains frequency, ie, 100 or 120Hz but it would also desensitise the unit to a degree because unless it was a half wave, the voltage will no longer reach zero for the magnetism to die away.

author: TheNewLab
author_rank: Frequent Contributor
timestamp: March 12, 2019, 01:57:04 am
content: Quote from: Specmaster on March 02, 2019, 01:03:31 amA lot of good advice and I am learning some stuff from this thread.Regarding the RCD and GFCI..I think the GFCI allows a larger amount of current before tripping. All I can say is what I have done.1st, I my work studio is in the US, 120v in an old building. where all the plugs are on a single breaker circuit. and only two outlets per room..very old. an electrician suggested I get a building Wiring Inspector unit like this:https://www.amazon.com/Amprobe-INSP-3-Wiring-Inspection-Tester/dp/B005E0XSR8/ref=sr_1_3?keywords=Amprobe+circuit+tester&qid=1552353823&s=gateway&sr=8-3plus a wattmeter to determine what actual current is being used.The wiring inspector has separate functions for GFCI and RCD. plus AFCI, another pain in the ass requirement in the US..but really makes sense.What I have done, is use multiple power strips. no surge protection or anything. then use surge protector strips at the end for devices that need it like computer, and even at the end just before the modern oscilloscope and arb generator.I after the many stuff I have connected, I inserted a watt meter at the receptacles to measure how much current and what voltage drops occurred. and turned everything on...I learned that modern test equipment doesn't consume all that much electricity.I few things I have to keep off, like the 2nd solder iron with everything else on.with the wiring inspector and everything disconnected. despite being old wiring. the ground wires ARE connected and are solid. the GFCI are good because the inspector tripped them.I know any qualified electrician who reads this would scream. but I have no choice. I run a number of extension cords around to get to places there are no outlets, however, I use HD outdoor extension rated at 12AWG and 14AWG. so there is no voltage or current drop. ie way overrated, and safely attached above the baseboards with Velcro wraps on the wall.This may sound crazy, but, the landlord is not about to do anything about updating the wiring. it is exempt form modern codes because the building is so old and he has not done any improvement or additions to the old wiring.I also have an isolation transformer for when needed.I do not have any problem with breakers tripping. I know that the building's wiring is solid and trips correctly with a GFCI. and I have protective surge protection where neededin summary: The switching a built-in heater circuit with a sub-panel is probably, I know a huge non-no in electrical wiring. but the MtF google-bithing run at 9A to 12A!!!! alone!!Now sorry pros, a have a 40 amp electric heater in one room, that I am considering disconnecting it an building a sub-panel with all the safety features and hope the landlord doesn't notice it