High Voltage tracks clearance - Page 1
Forum section: electronics / beginners

author: lmagalhaes
author_rank: Contributor
timestamp: September 25, 2019, 05:34:13 pm
content: Hi there,I've seen that I should keep a minimum of 2.54mm of clearance between high voltage tracks and, with through-hole TRIACs I was able to do that. But now I've been asked to switch to surface mount triacs and I'm having trouble in keeping that clearance with the space that I have in the PCB. Is it possible to decrease the clearance with some cuts between the tracks?Here's the before and afterhttps://imgur.com/a/6z6i6E4Since this is a TRIAC with a TRIAC driver and there's no neutral around, is it still needed to maintain the 2.54mm clearance?Thanks !

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 25, 2019, 07:41:23 pm
content: This is a situation where the correct terminology must be used - clearance is the distance through air between conductors while creepage is the distance along a surface. There are various regulatory documents that specify the distances required vs. voltage vs. application (medical has different requirements, for example) but a good site that covers the basics is:http://www.smpspowersupply.com/ipc2221pcbclearance.html

author: floobydust
author_rank: Super Contributor
timestamp: September 25, 2019, 07:50:02 pm
content: You need to have proper spacings at least four areas:-Between line and neutral-Between ground (PCB mounting spacers, bolts) and hazardous live traces-Between low voltage control electronics (opto LED) and hazardous live triac traces-Between each triac's channel hazardous live tracesFor up to 300VAC, overvoltage category II (2.5kV), creepage and clearance is minimum ~1.6mm on a pc board. Assuming no condensation and little dust, single-phase.If your solenoids have high kickback and there is no snubber, I try for >2mm.You need more heatsinking for the triac, there's not enough copper pour. It will heat up with an amp or two. You can rotate the triac 180 so closer to the opto, instead of long traces on the sides.In Altium, make a 1.6mm dia circle. Then drag it around your hazardous live traces to ensure you've got enough spacing. Setting up clearance rules in Altium only works half assed.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 25, 2019, 07:52:53 pm
content: Isolation requirements vary depending on voltage, application, environment, materials, and needed certifications. "A minimum of 2.54 mm clearance" is probably correct for some combination of those, but you need to figure out the right value for your application. I suggest you make sure your number is correct for your application. For example even within a given device isolation from live to neutral (for isntance) has different requirements from line to ground even though they are the same voltage.As far as definitions: clearance and creepage are two different things. Clearance is the physical distance through air between two conductors. creepage is the distance along a surface (e.g., along the PCB). The creepage requirements reflect the possibility of surface contamination or condensation allowing surface tracking.Creepage can be improved by using different materials, working in a less contaminated environment, or cutting slots in the PCB. To improve clearance you actually need to insert a barrier between the signals. One way to to this is to cut a slot in the PCB and then allow a plastic fin from the enclosure to poke through it. Now a potential arc would have to go around the plastic fin -- traveling a much longer distance.

author: Andreas
author_rank: Super Contributor
timestamp: September 25, 2019, 08:00:27 pm
content: Quote from: lmagalhaes on September 25, 2019, 05:34:13 pmreally? how much is the creep distance directly at the package of the TRIAC?The requirements are generally higher between "life" and "neutral"/"earth ground"as across a switch where the load limits the current.with best regardsAndreas

author: lmagalhaes
author_rank: Contributor
timestamp: September 25, 2019, 09:32:40 pm
content: Thank you guys for all the answers. These TRIACs are connected to 60W electrovalves working at 230VAC, consuming ~260mA. I did the math and determined that for the TRIAC used I did not need any heatsinks. The two TRIACs going to motors are through hole and have the legs with appropriate clearance + slots between them. Also, the leg layout made it really easy to connect to the optocoupler without compromising any rules. We have a lot of units in the field and all of them are working.But now, with the changes to a SMD package, the layout got really awkward. Not only that, but a smaller box was chosen, which meant reduced PCB size and that really made things even more awkward.I rotated the TRIAC and tried with a different layout - would this work ?https://imgur.com/a/jueDCmv

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 26, 2019, 12:05:06 am
content: just to be clear... coated? or not coated? because between 1.25mm and 0.4mm is lot of saving... the IPC2221 provided earlier is of much help...http://www.smpspowersupply.com/ipc2221pcbclearance.htmland i believe you dont just have to take care between life and neutral, but whatever voltage difference between adjacent traces, for example spikes generating nodes should be handled with care even if they are not connected to mains, imho. creating net class in Altium and then selecting suspected nodes into the class, set clearance for the class according to the IPC2221 may do the trick, ymmv.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 26, 2019, 12:33:34 am
content: FYI, IPC2221 is quite old and not a very reasonable calculation in the first place. IEC/UL 60950-1 is a better standard, typical for information technology equipment (ITE):http://www.creepage.com/You are only looking forfunctionalclearance/creepage here. The voltage across a TRIAC is transient limited (if subject to transient, the TRIAC merely turns on) so you use the line nominal peak voltage, or the TRIAC DC voltage rating, to determine distance.Note you need basic or reinforced insulation from mains to user-accessible low voltage (SELV) connections: basic if the connection is grounded, reinforced if floating. If you're using an opto to drive that TRIAC, it'll be made with this lead spacing in mind; keep similar distance elsewhere in the circuit and you're good.Don't overlook the option of adding routs/slots between pads, yes even underneath an SMT component. This can add cost (routs under 1 or 2 mm width typically add cost, since the fab has to use a smaller, faster wearing end mill to cut it), but that's only really a matter for large volume production.Tim

author: soldar
author_rank: Super Contributor
timestamp: September 26, 2019, 05:54:20 am
content: Quote from: T3sl4co1l on September 26, 2019, 12:33:34 amIndeed. considerations and regulations about the safety of persons are different and in addition to the safety of the equipment itself.Here's a start:https://www.ecma-international.org/publications/standards/Ecma-287.htm