double insulation  - Page 1
Forum section: electronics / beginners

author: Stefan
author_rank: Contributor
timestamp: March 03, 2015, 10:50:31 am
content: In Dave's teardowns I can sometimes see that wires that carry mains are additionally insulated by having them in a tube or similar.We we have (from the inside out):* live copper* insulation of the wire* tube* grounded metal chassisI'd like to learn more about that.* Why is this done?* When does it have to be done?* Can you put live&neutral in the same tube?* ...I tried to google, but all I find is this Class II for devices without a ground connection. This seems to be something different, as this run-wire-in-tube thing is done in grounded metal chassis as well.Any pointers are wellcome.Thanks,Stefan

author: Neilm
author_rank: Super Contributor
timestamp: March 03, 2015, 07:44:10 pm
content: If you used the term "double insulation" in your search terms, I'm not surprised you had issues finding the info. "Double Insulation" is a technical safety term to describe the quality of the insulation between any hazardous parts and the user. To confuse things, this insulation does not have to consist of two separate parts.To answer your specific question, it depends - I can think of 3 reasons for it.1) If the rating of the wire is not good enough for the application (be it mains or any other voltage) then an additional layer of insulation can be used to get the required rating.2) If there is a danger that the wire could rub against something and short out it needs protection. The additional tube is there to protect the wire from physical damage not shorting.3) There are several wires and putting a tube around the lot makes it easier to route.

author: SeanB
author_rank: Super Contributor
timestamp: March 03, 2015, 07:51:46 pm
content: The 2 layers are used as it is possible with a stranded wire for a strand to break from handling, and to then have the sharp point pierce the inner insulation layer. Having an outer separate layer means there is still a layer of insulation keeping it from contacting you. Your typical mains cable ( excluding the fake ones) uses 2 separate layers of insulation extruded over the bare strands, typically a white inner and then a coloured outer layer, then the whole lot is covered with the outer sheath. This gives the required 2 layers, and as well an indication when the coloured layer inside is damaged slightly during preparation. Inside equipment the outer is removed, so the sleeve is added as an equivalent layer. It is also added to terminals as a primary insulation as these are bare metal.

author: Zero999
author_rank: Super Contributor
timestamp: March 03, 2015, 08:46:11 pm
content: There are different types of insulation:Functional - only required for the device to work properly and doesn't provide any protection against electric shock. The enamel insulation on the thin copper wire in a motor is a typical example of this.Basic - required for protection against electric shock. The insulation covering a piece of electrical cable inside the outer sheath is an example of this.Supplementary - added on top of basic insulation to form double insulation. The outer sheath on a piece of mains cable is a typical example of this.Reinforced - thick, rugged insulation which is provides as much protection against electric shock as supplementary insulation on top of basic insulation. Multimeter test leads are an example of this.An appliance which is double insulated will have basic and supplementary insulation and/or reinforced insulation between all parts at dangerous voltages and the user.All appliances, need to have basic insulation, double insulated or not.

author: IanB
author_rank: Super Contributor
timestamp: March 03, 2015, 11:10:24 pm
content: Quote from: SeanB on March 03, 2015, 07:51:46 pmThis is country specific. Common practice for power cords in the USA is for just a single insulating layer giving a figure 8 cross section. If the cable has three cores including ground then it will have a ooo cross section. The double layer of insulation is only found in extension cords for power tools or for outside use.

author: bugs
author_rank: Contributor
timestamp: March 04, 2015, 10:02:53 pm
content: The basic standard for protection against electric shock is the IEC 61140. It states the fundamental rule as following: Hazardous-live-parts shall not be accessible and accessible conductive parts shall not be hazardous liveeither under normal conditions or under single-fault conditions. The first is done with basic isolation, the second can be in various ways.In Class-I equipment, fault protection is done with a protective conductor (PE). If the basic isolation fails, and the wire touches the metal of the chassis, the PE conductor has a resistance low enough to blow the fuse, and for that brief fraction of a second that it takes to blow the fuse it should also prevent the chassis voltage to become higher that a specific voltage (50Vac, 120Vdc).However, inside of the equipment there might not just be a hazardous voltage, but also what is called SELV: Safe Extra Low Voltage. This is basically any circuit with a voltage below 60Vdc or 42Vac. These circuits usually can find their way outside of the box to potentially uninsulated wires, because they are considered safe to touch. Therefore it is always considered a circuit that can be touched as if it is an exposed wire. That is why you always need double or reinforced isolation between SELV circuits and hazardous voltage circuits inside of the box, independent of it being class-I or class-II, etc. If the correct voltage rating wire is used the insulation is considered basic isolation. To prevent this wire accidentally touching a SELV part inside the box it is good practice to put it in an extra tube, or to thoroughly mechanically constrain the wire so the basic isolation of the AC wire can never touch any part of the SELV circuit. You need 2 failures before the SELV becomes live, the basic isolation must fail and the extra tube or mechanical constraint, I.E. a double fault.

author: Stefan
author_rank: Contributor
timestamp: March 05, 2015, 09:23:18 am
content: Many thanks for all the answers!That Dave likes test-gear teardowns, where there is lots of externally exposed SELV, explains the extra tubes (and also why he frowns when older gear is missing it).A shame that such standards are behind a pay wall. You'd think they don't want people to read the thing.

author: bugs
author_rank: Contributor
timestamp: March 05, 2015, 09:15:00 pm
content: http://www.google.nl/url?q=http://www.schneider-electric.com.au/documents/electrical-distribution/en/local/electrical-installation-guide/EIG-F-protection-electrical-shocks.pdf&sa=U&ei=gMH4VPHYM4O6Op3bgdgK&ved=0CBAQFjAC&usg=AFQjCNEYE4mr13_MPEH8k7nYljaWJSnhvQThis gives some idea of what is in the IEC 60364-4-41 which is based on the 61140. Standardization is a racket

author: Neilm
author_rank: Super Contributor
timestamp: March 05, 2015, 09:26:34 pm
content: Quote from: bugs on March 05, 2015, 09:15:00 pmA typical standard revision will take 3 to 5 years typically. TC66 (the committee responsible for IEC61010) meets every 6 months for 1 week, I think it has 20 or so members. I would think that it probably covers the cost of publishing / setting the final text.