Becoming crazy with EU LVD regulation in PCB and electric strenght - Page 1
Forum section: electronics / projects

author: Robotec
author_rank: Contributor
timestamp: May 26, 2017, 09:44:45 am
content: Hi, everyone , this its my first thread so hi to everyone!First of all i will explain the design characteristics of my project to make clear what it is and where it enters for regulation.It is a fixed electric/electronic installation in offices so it can`t be moved and the PCB board size has to be the smallest possible, it has plastic envelopment around all the product and earth and GND are tied(it has an isolated AC/DC power supply), and in theory the electronic`s space its waterproof, and the user can`t touch anything, its all ethernet controlled.So my doubts started when i had to implement the normative EN-60335 about electrical security in home electrodomestics and the IEC 60664 for insulated systems (coated PCB`s) which are the ones i assume i have to apply into the design.The problem comes when i want to use this connector to carry 2 com´s wires(serial 485),isolated logic power(+12V,GND-Earth) and 230V AC(line-neutral) (i know it shouldn`t carry all in the same connector but it was a design requirement):https://www.digikey.com/product-detail/en/molex-llc/1723101106/WM11592-ND/5344289in theory the electric strength of the connector is about 1800V but some electric test are about 5KV peaks in the normative, so one of my doubs are about the varistor, at the AC entry, shouldn't it shortcut when the peak tries to pass it and then the connector shouldn'tbe affected?In coated boards which is the minimum clearance and creepage between Line,neutral, earth and low voltage lines for this kind of product?Can the connector and electronic`s be filled with some kind of plastic isollator to upgrade it`s isolation capabilities?Thanks for your time.

author: Benta
author_rank: Super Contributor
timestamp: May 26, 2017, 12:17:29 pm
content: With a connector like this, the main limitation is creepage distance.As you have no protective earth, it's classified as Class II equipment. Creepage distance from live or neutral to "something you can touch" (in this case the RS-485 connection) is 8 mm.Live to neutral is 3 mm on the unfused side (towards the mains), on the fused side (inside your circuitry) it can be whatever your design needs.You'll most probably need to to remove one or two connector pins to get the required distance from mains to RS-485.

author: Robotec
author_rank: Contributor
timestamp: May 29, 2017, 08:22:09 am
content: Thanks for your answer Benta, but in my appliance the user cant touch the 485 is an internal bus between boards in a shielded cable with the shield connected to GND.Even in that case the creepage would be 8 mm?Right now im discussing changing to 2 connectors to get the required distances.Thanks for your help.

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 30, 2017, 08:57:17 pm
content: When I've had similar issues, I also have applied Benta's suggestion: namely, to leave one or two un-used, un-connected pins as buffers.That is, if available space allows it.

author: Benta
author_rank: Super Contributor
timestamp: May 30, 2017, 11:02:22 pm
content: Robotec, I've only given you the general rules on this, and I'm not prepared to go further.A clear-cut design, eg, a little power supply, you can do yourself.But a design like this, I strongly urge you to bring in a specialist/consultant. It will save your ass in the end.

author: Robotec
author_rank: Contributor
timestamp: May 31, 2017, 09:33:10 am
content: Thanks for all your answers, we are contacting with experienced consultants on this issue, anyway we decided to go for 2 connectors in the end.its better going for the safe side.Another question is how a unconnected pin would affect the isolation distances (creepage and clearance), wouldn't a unconnected pin act as an intermediate point between another two in a high voltage test?

author: AndyC_772
author_rank: Super Contributor
timestamp: May 31, 2017, 11:58:50 am
content: You can effectively add the creepage and clearance distances on either side of an unconnected pin. The thickness of the pin does not, of course, contribute to the effective separation.If you're worried, then removing the unused pin from *both* halves of the connector may be an option.IMHO if "carry 240V mains and low voltage signals on a common connector" is a design requirement, then "sod off" is the proper response.

author: Robotec
author_rank: Contributor
timestamp: June 05, 2017, 08:16:31 am
content: AndyC_772, thanks for the advice, at the end we "sod off" that requirement.and benta thanks for all your help,but i just became aware of your advice about the fused side where i can design whatever i need.Benta when you are talking about the creepage distance in the fused side, do you know where is the regulation about that?, i didnt found anything and it would be very helpfull not for this proyect but in the future to manage AC voltages inside my design because i wouldnt have to leave 8 mm in the worst case between the pins of the AC connectors(its a pain in the ass).

author: Benta
author_rank: Super Contributor
timestamp: June 05, 2017, 09:47:16 am
content: I'm not aware of any specific regulations on the fused side.But to give you an example, a TO220AB case has a creepage distance between the pins of 0.8...1.4 mm, and you'll find plenty of devices rated at 600 V in this package (SCR, Triac).Take min. 1 mm as a starting point and go from there.