240Vac MAINS circuit in metal chassis with floating ground - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: February 01, 2017, 12:58:54 pm
content: I have been asked to modify an existing PCB that is presently in production and is supposedly UL approved for use in the USA as UL 508 (Industrial control equipment)The grounding aspect of the board jumped out at me as possibly incorrect. So if your up on ground bonding and creepage please read on and let me know if If this is not quite right or I'm simply not interpreting the rules correctly.I'm under a NDA so I can't share every detail, however I can paint a pretty clear picture without disclosing anything confidential.Details:1) This device is powered from a 240V branch circuit protected with a 20A breaker2) The device is used to distribute 120/240V with taps at each point. Much like a standard power bar, however several of these are typically cascaded to allow tapping off of power at various locations while passing the power on to the next stage.3) The device is in a metal chassis4) The device uses flexible metal conduit for the power in and out connections; L1, L2, Neutral and safety Ground5) Within the metal enclosure a PCB is used to route the power (L1, L2, Neutral and Ground)6) A simple circuit on the PCB also taps into the 120/240V via a transformer to some user accessible low voltage circuitry (I don't see this as relevant, just noted for clarity)Now the perceived issues:a) The pass through ground connection on the PCB is not bonded to the chassis, the ground trace is treated the same as the other live and neutral connections and is essentially floating within the constraints of the PCB.note that the ground from module to module is continuous in that the flexible conduit is connected to the chassis (via sheet metal screws on the tab of the conduit connector), and the safety ground conductor passes through the PCB on the traces, however their is no intentional connection between these two ground paths.b) the creepage between the ground trace and the adjacent neutral is 3mm (119mil)Now if the ground trace on the PCB was bonded to the chassis, I think the 3mm creepage would be fine (more than adequate), however if UL interpreted the PCB as an ungrounded device then I think the ground to neutral connection would need to be reinforced or 5mmThis is what the board looks like

author: rstofer
author_rank: Super Contributor
timestamp: February 01, 2017, 05:52:01 pm
content: What "ground to neutral" connection? That is a MAJOR no-no!According to the National Electric Code, all exposed non current-carrying metal surfaces are to be grounded. Whether the flexible conduit is adequate is debatable. Some jurisdictions don't think so. First because it will have a relatively high impedance from a long, circuitous route of dubious conductance and second because the fittings rarely stay together over time.Were it me, I would have a solid ground connection to the chassis.Now, as to the ground path on the circuit board. I probably wouldn't do it that way. I want the output connectors to have a rock solid ground path. Tiny traces (less than the area of the required equipment grounding conductor, #12 AWG in this case) just don't impress me. You said something about "industrial". How much fault current is available? Typically, for residential, it is less than 10,000 amps, often only about 5000 amps. For some brands, you can look on the side of the breaker to find their fault current interrupt capacity. For industrial installations, the available fault current can be all over the map.

author: Zero999
author_rank: Super Contributor
timestamp: February 01, 2017, 06:14:32 pm
content: Quote from: rstofer on February 01, 2017, 05:52:01 pmI don't think there is a ground to neutral connection. This is most likely a poor choice of words by the original poster, who's more likely referring to the creepage between the ground and neutral traces.Quote from: Jester on February 01, 2017, 12:58:54 pmDoes the ground on the PCB actually connect to anything?I think it's right that the chassis isn't grounded via the PCB, which (as mentioned above) won't have sufficient cross-sectional area for a safety ground.3mm is enough creepage between a ground trace and a mains conductor.

author: wraper
author_rank: Unknown
timestamp: February 01, 2017, 06:30:17 pm
content: Quote from: rstofer on February 01, 2017, 05:52:01 pmIMO you made up that statement by yourself. Please give any sort of quotation to prove this. Quick example, take typical DVD player which has no ground connection but have full metal enclosure (yet somehow passed safety certifications).

author: Zero999
author_rank: Super Contributor
timestamp: February 01, 2017, 07:04:42 pm
content: Quote from: wraper on February 01, 2017, 06:30:17 pmYes, there's no requirement for the metal chassis to be earthed. If all exposed metal parts are adequately isolated from the mains, by double or reinforced insulation, then there's no need to earth the chassis.

author: Gyro
author_rank: Super Contributor
timestamp: February 01, 2017, 07:19:45 pm
content: Quote from: wraper on February 01, 2017, 06:30:17 pmThings like DVD players come under the heading of Class II double insulated appliances. That means, minimum of 8mm clearance between any live (or neutral) traces and case metalwork. double layer insulation on all mains wiring. Captive connection between the mains input and PCB so there is no possibility of it coming into contact with the case etc. Pretty strict.

author: rstofer
author_rank: Super Contributor
timestamp: February 01, 2017, 08:06:25 pm
content: Quote from: wraper on February 01, 2017, 06:30:17 pmNational Electric Code Article 250.110. I can't find a verbatim copy of the NEC (copyright issues, I suppose) but here is an article that describes the situation:http://iaeimagazine.org/magazine/2013/05/16/article-250-grounding-and-bonding/(half way down).Presumably the OPs project isn't "double insulated" and it doesn't appear to be cord connected. So, it's not an appliance, it is a piece of utilization equipment.In my view, all non current-carrying conducting surfaces must be grounded.Now, if it has a UL label, it must be installed according to the manufacturer's instructions and if UL doesn't want the metal grounded, so be it!NEC Article 110.3(B)https://jadelearning.com/jadecc/courses/UNIVERSAL/NEC05.php?imDif=2529

author: Vtile
author_rank: Super Contributor
timestamp: February 01, 2017, 09:03:45 pm
content: I withdraw my wondering.

author: Jester
author_rank: Frequent Contributor
timestamp: February 01, 2017, 10:45:33 pm
content: Quote from: Hero999 on February 01, 2017, 06:14:32 pmThere is no ground to neutral connection, my point was that the creepage between these two traces is presently 3mm and 5mm is required for reinforced

author: Jester
author_rank: Frequent Contributor
timestamp: February 01, 2017, 10:52:45 pm
content: Quote from: Hero999 on February 01, 2017, 06:14:32 pmDoes the ground on the PCB actually connect to anything?Only to the input and output connectors on the PCB, and from my understanding it must be bonded to the chassisI think it's right that the chassis isn't grounded via the PCB,which (as mentioned above) won't have sufficient cross-sectional area for a safety ground.Actually I think the traces do have have sufficient cross-sectional area for a safety ground (the trace is 3.3mm wide on both top and bottom layers and 2 oz copper is used so that equates to a 6.6mm wide trace), so with 20A flowing would rise < 20 deg C.Why do you think the ground trace on the PCB should not be bonded to the chassis?

author: Jester
author_rank: Frequent Contributor
timestamp: February 01, 2017, 11:07:31 pm
content: Quote from: Hero999 on February 01, 2017, 07:04:42 pmThis highlights my point, IF the chassis was grounded, only basic insulation would be required between the ground and neutral traces. However the ground conductors (and ground traces on the PCB) are not intentionally grounded within this device so the only ground connection to the chassis is via the metalic flexible conduit, and as mentioned earlier (rstofer) some safety agencies would either not accept this as a viable ground or frown upon it. So IF the conduit is not considered a ground that makes the chassis ungrounded and an ungrounded chassis requires reinforced insulation (5mm), while the present neutral to ground trace creepage is only 3mm.I think the simple solution is to simply bond the ground trace to the chassis.I know the safety agencies can be pretty picky about the details of the ground bond screw, washers etc. so I'm not sure if a #6 machine screw through a large pad to a metal standoff attached to the chassis would satisfy a picky inspector?Perhaps a better approach is to add one more terminal and connect that to the ground traces on the board and to the chassis via a sort piece of green or bare #12 and bond that with a machine screw, lock-washer etc.

author: Zero999
author_rank: Super Contributor
timestamp: February 01, 2017, 11:30:35 pm
content: Quote from: rstofer on February 01, 2017, 08:06:25 pmWhether the metal case needs to be earthed or not, depends on how well isolated it is from the live parts. In double insulated equipment, the case is designed so it can't come into contact with live parts.https://www.tlc-direct.co.uk/Book/5.8.1.htmOf course, we don't know whether this applies to the original poster's equipment because we don't have enough information.

author: Jester
author_rank: Frequent Contributor
timestamp: February 01, 2017, 11:38:00 pm
content: Quote from: Hero999 on February 01, 2017, 11:30:35 pmWell to answer your question there is adequate clearance and creepage from the board and all the components on the board to chassis (>250mils), however that takes us back to the ground trace that is only 3.3mm from the neutral trace, and that violates the reinforced (5mm) rule.

author: richard.cs
author_rank: Super Contributor
timestamp: February 02, 2017, 11:12:15 am
content: If the case is sufficiently isolated from live parts (double or reinforced insulation) then the case does not need to be earthed. If the earth does not connect to the case then why is the creepage distance to the earth conductor relevant? Surely only functional insulation is needed here?

author: Jester
author_rank: Frequent Contributor
timestamp: February 04, 2017, 03:11:42 pm
content: Quote from: richard.cs on February 02, 2017, 11:12:15 amWell the case is sort of grounded via the metalic sheath of the flexible conduit.All of the circuitry on the board is spaced at least 5mm from the chassis (so that meets the reinforced rule). However the ground wire from the circuit breaker panel connects to a trace on the board that is spaced 3.3mm from the neutral, so one could now argue that the case (via the ground sheath) is actually only 3.3mm from the neutral or less than reinforced. I realize it's a bit of a circuitous argument because if the conduit is good enough to ground the enclosure its good enough to allow for single insulation and if it's not a good enough to ground the chassis then it does not come into play for the less than 5mm neutral to ground creepage.Another way to look at is if I were to perform a Hi-Pot test between ground and all the live and neutral connections, for reinforced 240V (3000Vrms), 3.3mm creepage is marginal

author: SL4P
author_rank: Super Contributor
timestamp: February 04, 2017, 10:18:48 pm
content: Question: With the ground trace being the same width/weight as the live and neutral paths... which 'fuse' will go first in event of a failure?I'd be bonding to the case!

author: Jester
author_rank: Frequent Contributor
timestamp: February 05, 2017, 01:33:35 am
content: Quote from: SL4P on February 04, 2017, 10:18:48 pmAnother good point. Well in theory the circuit breaker back at the box before the traces on the board that can support 20A.That being said I think the only logical thing to do is bond the enclosure to ground. Its the simple and safe thing to do.