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I've become fairly familiar with the clearance and creepage requirements of UL/IEC/etc 60950, 60730 and derived/related standards. I've just realised, though, that they only refer to distances through air (clearance) or along a surface(creepage). In fact, solid insulation is specifically used as a barrier to increase the distances. Does this mean, then, that the PCB itself is considered solid insulation, and completely removed from the clearance rules? Can I safely have 240VAC on one side of the PCB and user accessible low-voltage circuits on the other, provided there aren't any vias in the vicinity?

Extending the question, what role does the thickness, dielectric strength, etc play? If a thing is considered "solid insulation" is the only requirement that it passes the hipot testing, and if so, at what voltages?

UStralian
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3 Answers3

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PCBs are treated by the IEC 60950-1 "separately" as for solid insulation, that is when the path of discharge passes through a solid object (i.e. the substrate / prepreg).

  1. The main requirement is that the thickness must be >= 0.4 mm: that barely fits the 4-layer pcb with standard thickness of 1.6 mm. A simpler 2-layer PCB is evidently ok.
  2. Table 2R of the standard indicates Type Tests, that have the purpose of assuring some quality in the fabrication: PCB manufacturers comply with this, although it may be difficult to get the right documentation. This type of tests was required for insulating layers other than the standard material including pre-preg, when cured below 500 deg.
  3. Then the electric strength test is applied almost to all cases, and it is a routine test (on all samples). This test can be carried out with AC sine waveform or at DC with value equal to the peak AC. Although the test is carried out by applying the voltage for 1 minute, when it is used for routine test (as for PCBs) the standard allowed to reduce the duration to 5 seconds, and the amplitude by 10% (when used amplitude is that of Table 5C). I fact, there are two criteria: Table 5B based on peak working voltages (so the maximum repetitive value including DC ad AC components of the operating waveform); Table 5C based on required withstand voltage, thus distinguishing between basic insulation and reinforced insulation (factor 1.5x approx).

Going to the question on 240 Vc: yes, you can; I used 400 Vac on an old 2-layer pcb (old = 35 years ago, so now quality of materials has improved).

Going to the last point: thickness must be >= 0.4 mm, no other requirements; if a material is better, well, you have a better margin but you must pass the electric strength test & you must have 0.4 mm. If your material is able to work well with only 0.1 mm, nope: if it gives insulation for safety it must comply to 0.4 mm; if it were just for functional insulation, you can do what you want.

andrea
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    Brilliant, exactly what i was looking for. Couple of years after I finished that project, but nevertheless, will be useful for anyone doing something similar. – UStralian May 27 '21 at 08:11
  • Yes, indeed LOL. If you have other questions, just ask, now I am in: I work among others on product engineering, especially test & meas products, and in general electric tests in various sectors. So, I have an overview of various standards. That does not mean that I know how to make things work! ;) – andrea May 27 '21 at 15:38
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Can I safely have 240VAC on one side of the PCB and user accessible low-voltage circuits on the other, provided there aren't any vias in the vicinity?

It's best to separate high voltage with other secondary voltage components. If you need them on the same board, ensure the trace is at least 5mm (forgot the actual number) away. Also the components nearby when bend should not get close to the primary voltage.

That being said, board thickness is around 1.6mm. So no, you can't mount them on reverse side.

Jason Han
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    Consider, for example, a transformer. Does it have 5mm between the primary and secondary windings? No, but it does have insulation between them - enough to pass the hipot test for whatever its isolation requirements are. – UStralian Aug 04 '17 at 23:52
  • PCB safety design is not component design. The rules are there for it to pass safety regulations. Without passing safety, the product can't sell in many countries that require them. – Jason Han Aug 05 '17 at 00:10
  • Yes, I know. I'm not asking if I can break the rules, I'm asking for help understanding the rules. The clearance rules say clearance through air, and around solid insulation. Is the pcb solid insulation? – UStralian Aug 06 '17 at 01:33
  • You can read more here: https://forum.mysensors.org/topic/4175/clearance-creepage-and-other-safety-aspects-in-mysensors-pcbs From what I understand in this article, you may use your PCB as an insulator to increase your creepage distance. However the insulator must be 2.5mm. – Jason Han Aug 06 '17 at 02:27
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Extending the risks to AC_coupling through the FR-4. 1mm trace width, 10cm (4") long in the low-level side of the PCB, has

$$ Capacitance = Eo * Er * Area/Distance $$

C = 9 e-12 Farad/meter * (5 = FR-4 Er) * 1mm * 100mm / 1.5mm (1/16th ")

C = 9e-12Farad/meter * 5 * 67mm * [1meter/1000mm] = 2.5 e-12 = 2.5pF

Using I = C * dV/dT, and knowing the slewrate of a CLEAN 60Hz at 120vac is 377 * (1.414 * 120) = 64,000 volts per second, we find the injected current is

$$ Iinjected = 64,000 * 2.5pF = 160,000 picoamps or 160 nanoamps or 0.16uA.

If the 120vac is FREE of any spikes.

Microwave ovens will up that slewrate by 100X, if not more, in which case you and 16 uA of spikes, occurring 120 time per second.

Please reconsider the casual placement of any circuitry (or logic) under or near the 120VAC power.

analogsystemsrf
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  • Try pressing _alt gr_ + _shift_ + * and tell me what you get. You should get ×. And tell me what you get if you use \\$\frac{1}{2}\\$. You should get \$\frac{1}{2}\$ – Harry Svensson Aug 04 '17 at 04:56
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    Sorry, I understand the capacitive coupling concerns, and can easily mitigate them in the application. I'm not suggesting anything like 100mm overlap, just a couple of sensing components. I'm asking specifically about compliance - if I can make the circuit correctly do what I need, will it fail 60950 solely based on only 1mm clearance, or does the fact that it's solid insulation solve that problem? – UStralian Aug 04 '17 at 16:15