Creepage requirements for mains?

[sparkydog]

I'm trying to understand creepage requirements for mains voltage, but I've seen possibly conflicting information.

Here, someone said that boards intended to handle mains should be rated for 1.5kV. Going by IEC 62368-1, that would seem to indicate 5.6 mm creepage best case (Pollution Group 1 a.k.a. a board sealed against contaminants and condensation). However, here, someone said that 240VAC only needs 1.6 mm accounting for overvoltage.

Which of these numbers is reasonable? Is IEC 62368-1 Table 17 accounting for expected overvoltage? If not, how does one determine what voltage to use for creepage per Table 17? (For bonus points, does anyone know the source of the second number?)

[Benta]

You need to differentiate two things:
One is the creepage phase-neutral or phase-phase. That's relatively lenient compared to:
Two, mains-consumer, which is much harsher (for good reasons).

If you keep those two scenarios in mind, I find it easier to understand the isolation requirements.

[TimNJ]

I’ll check IEC62368-1 tomorrow when I have access. Off the top of my head, I know the medical standard requires 3mm between line and neutral (before fusing) for 240Vac nominal mains. This is inclusive of expected transients on the line. (I believe over voltage category (OVC) II, and I forget which pollution level.)

Practically speaking, a reasonable rule of thumb for PCB spacing 1mm/KV, which of course comes with a number of caveats, including actual CTI of the board among other things. Lightning surges can be 1-2KV peak so using the rough rule of thumb above, 2-3mm is reasonable for most installations.

[T3sl4co1l]

The smaller figure will be functional only.  Perhaps transient breakdown is even acceptable; it is strictly unacceptable for safety purposes (i.e. where it can jump to a user), hence the additional distance in that case.

Tim

[coppercone2]

what pollution group is a face hugger carcass?

the best way to learn is to take apart equipment used by real customers in real conditions to see the expected dust load. its usually pretty bad

[Zero999]

There are three classes of insulation:

1) Functional: phase to phase or neutral.
2) Basic: phase to earth or another part which has basic insulation from the user.
3) Reinforced: phase the user.

Reinforced and basic insulaion are safety crictical and functional insulation is not.

Reinforced insulation requires twice the separation as basic insulation.

The minimum distance between tracks on a PCB depends on the insulationn class, voltage, environment and the tracking index of the PCB.

Solder resist is too thin to count as providing any additional protection, but there are conformal coatings available which do.

[TimNJ]

Check B.4.4.1 and B.4.4.2:

B.4.4.1 Clearances for functional insulation
Unless the clearance for functional insulation complies with:

- the clearance for basic insulation as specified in 5.4.2 ; or
- for ES1 and PS1 circuits used in pollution degree 1 and pollution degree 2
environments , the clearance for basic insulation for printed wiring boards as specified in
IEC60664-1:2007 , Table F.4 ; or
- the electric strength test of 5.4.9.1 for basic insulation,

a clearance for functional insulation shall be short-circuited

For line-to-neutral/line spacing, you don't strictly have to meet the spacing for basic insulation, but you would at least need to pass the electric strength test of 5.4.9.1. So unless, you absolutely can't meet the spacing, you need to comply with the requirements for 'basic insulation'.

Clearance:

Option 1: Select the row in Table 10 corresponding to 2,000V for AC mains <250Vac nominal  (according to 5.4.2.2) --> 1.27mm for all pollution degrees. For operating altitudes above 2000m, multiply the clearance by the factor in Table 16. For example, for 5,000m operation, 1.48 * 1.27mm = 1.87mm

Option 2: There is an alternative approach for determining clearance per Annex X (Table X.1, X.2). It's a bit confusing to me because the numbers don't really match up (stated transient voltages, and the resulting clearance requirement). Nonetheless, according to Table 12, the transient voltage for OVC II (normal appliances/equipment) is 2,500V, a departure from the 2,000V stated in 5.4.2.2. The clearance requirement for basic insulation is 2.0mm. For 5000m operation, 1.48 x 2.0 = 2.96mm.

(Actually, 2.96mm is the exact requirement from the medical standard IEC60601-1 @ 5000m, so it must be somewhat unified.)

I guess under certain circumstances it may be advantageous to pick Option 1 vs Option 2 (and vice versa), if you are constrained on space. *shrug*


Creepage:

The creepage requirement only considers the RMS working voltage (generally speaking, the max nominal nameplate voltage, e.g. 240Vac).

1. Assume Pollution Degree 2, unless your equipment is intended for outdoor usage -- Then I think it defaults to Pollution Degree 3
2. Assume PCB CTI is material group IIIa or IIIb

Creepage requirement --> 2.5mm for PD2/CTI = IIIa/IIIb


Then, under 5.4.3.1, note the line which says:

If the minimum creepage distance derived from Table 17 or Table 18 is less than the minimum clearance, then the minimum clearance shall be applied as the minimum creepage distance.

So, in this case, if we take 2.96mm  as the minimum clearance, then the minimum creepage must also be 2.96mm minimum.

[sparkydog]

You need to differentiate two things:
One is the creepage phase-neutral or phase-phase. That's relatively lenient compared to:
Two, mains-consumer, which is much harsher (for good reasons).

By "mains-consumer", do you mean the distance between AC and DC? Or between electrical components and a human/animal? (For the latter, a lot of the stuff I'm designing is meant to be in metal boxes with strict disconnect before servicing instructions. Still, something to keep in mind...)

what pollution group is a face hugger carcass?

How conductive is it? 😉 PG3 includes metal filings, IIRC, so it's probably PG3. (Replace "face hugger" with "mouse", or "spider", and that's actually a legitimate question. I've seen dead mice in breaker boxes...)

The minimum distance between tracks on a PCB depends on the insulation class, voltage, environment and the tracking index of the PCB.

Solder resist is too thin to count as providing any additional protection, but there are conformal coatings available which do.

Yes, I'm aware of both of these, hence why Table 17 is a table. Conformal coating can change your Pollution Group.

In this instance, I'm worrying specifically about PCB design, so I guess "functional" and possibly "basic" insulation. The others affect product design but have less of a direct impact on PCB design.

Do I understand correctly that Table 17 is adequate for functional separation, with the proviso that e.g. a lightning strike might fry the board, while the advice to insulate for kV is more applicable to ensuring such a spike can't jump from the product to a nearby person?

[coppice]

To clarify what others have said, the creepage required by safety standards are not based on theory. Its based on practical experience. The theory of nice clean products in a clean dry atmosphere doesn't help much for making safe products in moist dirty real world conditions. Can we use a theoretical worst case? Not really. Dirt and moisture can be really creepy. So, is 4mm across a PCB surface enough for 230V mains? People used to think so, and that was a safety spec. The rate at which that turned out to be insufficient proved too high for people's comfort, so it was increased to 6mm across a board's surface. It all comes down to practical compromise.

[Benta]

You need to differentiate two things:
One is the creepage phase-neutral or phase-phase. That's relatively lenient compared to:
Two, mains-consumer, which is much harsher (for good reasons).

By "mains-consumer", do you mean the distance between AC and DC? Or between electrical components and a human/animal? (For the latter, a lot of the stuff I'm designing is meant to be in metal boxes with strict disconnect before servicing instructions. Still, something to keep in mind...)

I apologise for poor phrasing.
By "mains-consumer" I mean the isolation to humans/animals/other living things.

[Zero999]

In this instance, I'm worrying specifically about PCB design, so I guess "functional" and possibly "basic" insulation. The others affect product design but have less of a direct impact on PCB design.

You're mistaken. Reinforced insulation is often required on PCBs, for example between the mains and secondary side of a switched mode power supply.

Do I understand correctly that Table 17 is adequate for functional separation, with the proviso that e.g. a lightning strike might fry the board, while the advice to insulate for kV is more applicable to ensuring such a spike can't jump from the product to a nearby person?

I don't have a copy of that standard handy.

No, there is no way to ensure a spike from lightning can't jump to a human. If the standards are met, it just covers your back.