Relay trace isolation - Page 1
Forum section: electronics / projects

author: bigredlevy
author_rank: Newbie
timestamp: January 12, 2021, 10:54:23 am
content: TL,DR: Does this relay (and footprint) meet the requirement for isolation distance?I'm working on a PCB for relay control of a 240V AC 5A load.The relay was chosen for convenience of purchase. Datasheet (2 form C):https://www.jaycar.com.au/medias/sys_master/images/images/9484866846750/SY4008-dataSheetMain.pdfThe required footprint is LY20 from the Eagle Pcb relay library.The traces of the footprint have only ~1.37mm clearance between the NO and NC pads.IPC2221A (2003) states that 339Vp AC across uncoated external conductors (B2) requires an isolation distance of 2.5mm.Does using this footprint de-rate the relay to 300Vp (as per IPC2221A)?Furthermore, as the distance between edges of holes to suit the pins are still only 2.4mm apart, drawing my own footprint with offset annular ring pads would still not meet the IPC2221A requirement.Is the PCB mount version of this relay therefore unsuitable for 240V?Is there an Australian standard that states a different isolation distance for this voltage?

author: capt bullshot
author_rank: Super Contributor
timestamp: January 12, 2021, 11:02:04 am
content: For safety reasons, you'd have to matter about the clearance between the contacts and the coil. You haven't shown this here.Typical standards (like IEC 61010) want a minimum distance between the isolated side and the mains related side, depending on the application and other constraints. If you've got at least 5.5mm, you're mostly on the safe side, 3mm might also be sufficient.Otherwise, the clearance between the NO and NC isn't that relevant for this kind of safety. If you'd have to declare conformity to standards you might want to have 1.5mm here, but depending on constraints and testing less would be OK too.Don't know the standards you cited, first of all check if they're applicable for your circuit. If not, don't care. Of course, the clearance from these standards might be different than the ones I'm used to.In general, if you don't have to be conform, 1.37mm would be OK for the application you describe.

author: bigredlevy
author_rank: Newbie
timestamp: January 12, 2021, 11:32:19 am
content: Isolation between the contact and coil pads of the footprint is 3.9mm.Isolation to the ELV side of the PCB is 3.8mm due to the trace size I have selected.IPC-2221A is the only documentation I can find freely (pirated??) available.http://www-eng.lbl.gov/~shuman/NEXT/CURRENT_DESIGN/TP/MATERIALS/IPC-2221A(L).pdfFrom wiki: IPC is accredited by the American National Standards Institute (ANSI) as a standards developing organization and is known globally for its standards. It publishes the most widely used acceptability standards in the electronics industry.I will try to find a copy of IEC 61010 as you suggested.It just seems strange that the relay would be rated for this voltage while the package is not conformant.Could it be that use in an insulated enclosure would make it conformant?

author: capt bullshot
author_rank: Super Contributor
timestamp: January 12, 2021, 12:36:22 pm
content: Quote from: bigredlevy on January 12, 2021, 11:32:19 amDepends on constraints, standards (especially which ones are to be applied), and sometimes the mood of an auditor.The very same component can have different ratings depending on the applied standard.Often one can find clearances on component packages that are smaller than applicable standards, this can be bypassed by testing or risk analysis or whatever.QuoteThat would help if you have to deal with environmental pollution, but not with too narrow clearances.Anyway, if your project is a one-off for your own (or very restricted) usage, I'd recommend not to deal with standards. If you are liable for the product, consult an expert.

author: Benta
author_rank: Super Contributor
timestamp: January 12, 2021, 07:58:48 pm
content: A way to increase creepage distance (often used with optocouplers) is to mill a slot in the PCB between the affected pads. This makes the distance into an air gap (instead of creepage), which has different specs.