Clearance between PCB tracks with 300V of potential difference?

[Ice-Tea]

To further the analogy: you're now the criminal that's asking what the chances are of getting caught.

Standards are drafted by experts. They see the iceberg, where you only see the tip. As a result, they may seem opaque but mostly it's just good and knowledgeable People trying to be precies. Granted, it's a bit of a jungle though.

[capt bullshot]

Has any company ever been fined, or forced to do a  total product recall, due to a standards infringement.

Never heard of such a case ... Wait, that's not true, I remotely remember reading of companies beeing forced to recall products (I don't think of the burning Samsung Smartphone now), but this never happens "out of the blue", there's always been an incident or someone volunteering to find flaw on a products. A competitor doing so comes to my mind ...

But there's no such thing as a mandatory and preventive check of your product before you can put it to the market. You declare it conforms to the rules. You can do so by just cheating, no one will notice.

By doing so, you take all the responsibilty and liability on your own risk. And now for the special part: If you made your product according to applicable standards and you have proof of this, then in case of damage, it is believed the it's not your fault by default. Otherwise you'll have to prove the particular damage isn't your liability due to bad product design. Disclaimer: IANAL, just an EE.

What is well known: customs rejects importing of goods that aren't declared to be conform. But they do not check if the conformity is real or cheat.

[capt bullshot]

Does it ever occur to you that asking about these things is analogous to asking why gambling is illegal?  Like, "what is the actual basis for that statement"?  Because that's what you're doing here, gambling with your customers' safety.  You're, at best, an incompetent engineer, and at worst, an unethical and irresponsible one.

Don't be so harsh :; It's the way things work in a small company (at least ten years ago when I worked for a quite small company). It wasn't even considered to research all the applicable standards, too much effort ... So why question the rules if you don't even know them?

Now I'm working in a different context, nobody wants to take responsibility, so obeying applicable standards is the way to go.

[ocset]

Thankyou , I must now tell of the story of a fantastic little company called Cyden. 

They pioneered hair removal by high intensity light application. They developed an original  portfolio of fantastic hair removal products which were brilliant, absolutely perfectly safe,  and had started to gain export sales.   

Then one day, a huge  engineering corporation phoned them up and asked them if their product conformed to all of hundreds upon hundreds  of quoted  regulatory standards.
The huge corporation then told Cyden that they were going to bring Cyden’s products into their standards lab…and rip them apart to  see if they passed  absolutely all sub-clauses of every single regulatory  standard.            
The huge corporation told Cyden that if its products did not conform to absolutely  all  standards from top to bottom, then they would be forced to do a total product recall from  several of the  countrys  in which this huge corporation resided…as well as receiving a big fine.

However, the huge corporation gave Cyden an alternative……   

If Cyden agreed to sell itself  off to the huge corporation concerned then  none of the standards checking would happen.
And indeed the company sold itself off………and still makes great products, -but it  is simply now a little employee of the big corporation, and gets payed a pittance of a wage. They are certainly now not allowed to put their own name on their own products…the name of the huge corporation  goes on there instead.
Their products were entirely safe and reliable, but   the huge corporation has an enormous department full of standards  checking employees….and there was no way that Cyden could guarantee being able to conform to the letter of all that  enormous sea of standards.
The same thing happened with uvintegration.com…..now sold off to an overseas company, totally.
..and scores of others have gone the same way.
Very often, the gaffer of the small company being bought out gets given a bit of a “nugget”, to sweeten the disappointment, shall we say.
After having the "sweetener" forced upon him/her, the gaffer is probably less inclined to speak about what happened.
Certainly, speaking out about it  might be viewed rather dimly by the buyout company....the gaffer's position in the newly acquired "subsidiary" mght come under "review", shall we say.

Then you end up with a situation like in the UK where 66% of all industry (>500 staff) is foreign owned. Quoted from Baroness Neville-Rolfe, Uk minister of business in 2016

 

[ocset]

1,000V transients on the mains are so common as to be basically uninteresting and 5,000V is not particularly remarkable.

Thankyou , please consider the following capacitor/diode being placed across the output of a mains rectifier. (no other circuitry involved)
How often would you expect this capacitor’s voltage to be taken above 450V? (I  am speaking about connection to the mains in UK or Europe or USA or Australia or New Zealand)

[T3sl4co1l]

Note that RFN5TF8S avalanches around 900V:
http://rohmfs.rohm.com/en/products/databook/datasheet/discrete/diode/fast_recovery/rfn5tf8s.pdf
(Typically, breakdown is a modest percentage above Vrrm.)

Assuming no series resistance beyond the surge network, the excess voltage can be calculated.

IEC 61000-4-5 surge is based on a 20uF capacitor.

Assuming charge equality at the waveform peak, a 1.5kV (line-line) surge will divide into 33uF giving 933V.  The diodes will feel it.

Tim

[ovnr]

Honestly, I'd never put mains on a MCPCB no matter what. Not saying it can't be done, but it's iffy and outside my comfort zone. I always go for full reinforced insulation specs (>= 4mm clearance) in any event.

Filtered, clamped DC at 300V on a MCPCB? Sure, not a big deal.

I don't think it was entirely clear if your product is single-board, or if you have a MCPCB for the LEDs and a separate board for the power supply?

[ocset]

I don't think it was entirely clear if your product is single-board, or if you have a MCPCB for the LEDs and a separate board for the power supply?

The power supply is linear led current regulators feeding in to the multiple led banks on the pcb.

IEC 61000-4-5 surge is based on a 20uF capacitor.

How common are these type of transients?
How often do they occur?

Assuming charge equality at the waveform peak, a 1.5kV (line-line) surge will divide into 33uF giving 933V.  The diodes will feel it.

Thanks, but i just cant believe that the vast majority if mains transients would be able to take the capacitor of Reply #31 above 450V.

[Gyro]

IEC 61000-4-5 surge is based on a 20uF capacitor.

How common are these type of transients?
How often do they occur?

You're approaching that question from completely the wrong direction. From the perspective of a cautious product designer, the question should be 'How often can I afford these transients to occur?' It sounds as if your current answer may be 'less than once'.

[Ice-Tea]

IEC 61000-4-5 surge is based on a 20uF capacitor.

How common are these type of transients?
How often do they occur?

[/quote]

1) Once is enough
2) 1 minute google in attachment.

Assuming charge equality at the waveform peak, a 1.5kV (line-line) surge will divide into 33uF giving 933V.  The diodes will feel it.

Thanks, but i just cant believe that the vast majority if mains transients would be able to take the capacitor of Reply #31 above 450V.

Belief doesn't really hold a lot of weight. And once is enough.

[ocset]

Thanks Iced Tea.....thats great data on transients.   

However, it doesnt allure to how much energy is in each of those transients.

There is no way of knowing how many of those transients would cause the 33uF capacitor of post #31 to go above 450V.

Its presumbaly possible that none of the transinets of that graph could take the capacitor of Reply #31  above 450V? 

 

[ovnr]

I think something you're forgetting is that the cap isn't perfect. If you get a nice, fat transient down the line, you're going to have pretty short rise times. The capacitor ESR will prevent it from absorbing the whole thing; you can still get higher voltages on the board, even though the cap isn't charged above 450V.

Also, keep in mind that it's not the arc-over from the transient that's going to burn your house down. It's the newly ionized (... and possibly carbonized) channel that allows the line voltage to flow across your board, causing much excitement.


I can't help but feel like your attitude to the problem is a bit wrong too. "Can't we cross our fingers and hope that nothing goes wrong?" isn't the safety-minded approach; you should instead say "What can we do to make sure this thing doesn't kill someone even under adverse conditions?".

The standards are there for a reason. If you follow them, no slimy lawyer can go "blah blah negligence" later on. If you skimp on things... well.

[Ice-Tea]

As Tesla stated: the standard is modelling a surge with a 20uF. Again: there's a reason they do this, in this case probably because it approximates real life. So it's easy enough to deduce the occurence from the graph I found. Which is, by the way, annecdotical. Could be a lot worse. Or not. To eliminate that kind of guesswork there are... standards!

[ocset]

Thanks,  incidentally  I dont think there is danger  of fire in an enclosure full of non-flammables.
FR4 PCB isnt flammable.

I once accidentally plugged a three phase input into the output connector of a prototype 10kW electric drive.
The explosion was quite earth shattering.
It was like being next to a cannon
The PCB was melted and contorted beyond belief, but there was no flame, and no fire.

[Gyro]

You are frighteningly complacent! 

[ocset]

Thanks, thing is, look inside any offtheshelf offline PSU you like, you'll see just a mov transient protector, -nothing more..give that a good few transients...and that MOV fails......and then your product has no transient protection.

And the failed mov won't blow the fuse, because manufacturers these days are putting movs upstream of the input fuse.

  

[T3sl4co1l]

Most things don't have a MOV at all, and survive just fine.

A keen observer would take this and run:

IEC 61000-4-5 surge is based on a 20uF capacitor.

Assuming charge equality at the waveform peak, [the] surge will divide into [capacitance] giving ...

33uF is insufficient.

What about a value much greater than 20uF?  What about 100uF, or 1000uF?

Indeed, check this out: https://zerosurge.com
How's that work?  Simple: https://www.google.com/patents/US4870528

Looks like they use a stage or two of 220uF to deal with it.  Series inductance to help even more.

It won't work for unimaginably large surges, or bursts of several surges together, that charge the capacitors too far.  But it will help with the most common events, which are also comparable to, or smaller than, the standard it's tested to.

Put another way: to a certain extent, it's gaming the system.  Why should the protection device withstand any more than what the standard specifies?

Now consider what "any offtheshelf [sic] offline PSU" has inside it -- most I see don't have MOVs at all.  They survive just fine!

I am now a bit curious about the smaller supplies.  Probably, resistance in the CMC goes a long way, reducing the impact.  Or current inrush, for which some don't even use an NTC at all, but a fusible resistor (usually 4.7 ohm).  Make that resistor surge-proof as well, and there you go.

Tim

[Zero999]

When at outer layers without encapsulation, you need 1.6mm creepage and 1.5mm clearance (for functional insulation)

1.5mm of clearance and 1.6mm of creepage sounds overkill for functional insulation.

Why did they pick such impractically large figures? The creepage distance between the pins of many power semiconductors is much less than that. It's not possible to use a 400V TO-220 TRIAC and maintain 1.5mm creepage on the PCB, without bending the leads.

I can understand having a large creepage distance, on the PCB, before the fuse, as failure can result in fire, but afterwards seems unnecessary!

[capt bullshot]

When at outer layers without encapsulation, you need 1.6mm creepage and 1.5mm clearance (for functional insulation)

1.5mm of clearance and 1.6mm of creepage sounds overkill for functional insulation.

Why did they pick such impractically large figures? The creepage distance between the pins of many power semiconductors is much less than that. It's not possible to use a 400V TO-220 TRIAC and maintain 1.5mm creepage on the PCB, without bending the leads.

I can understand having a large creepage distance, on the PCB, before the fuse, as failure can result in fire, but afterwards seems unnecessary!

You can use smaller clearances and get e.g. CE compliance certification from a notified body. It just involves more effort like testing what happens if these components fail. Test setups etc. are well-defined. You'll have to consider other failures too, e.g. what happens if the reduced clearance at your triac fails and a heat source that is intended to work on low duty cycle gets full power then? This can easily start a fire if no other countermeasures are provided.

[Zero999]

When at outer layers without encapsulation, you need 1.6mm creepage and 1.5mm clearance (for functional insulation)

1.5mm of clearance and 1.6mm of creepage sounds overkill for functional insulation.

Why did they pick such impractically large figures? The creepage distance between the pins of many power semiconductors is much less than that. It's not possible to use a 400V TO-220 TRIAC and maintain 1.5mm creepage on the PCB, without bending the leads.

I can understand having a large creepage distance, on the PCB, before the fuse, as failure can result in fire, but afterwards seems unnecessary!

You can use smaller clearances and get e.g. CE compliance certification from a notified body. It just involves more effort like testing what happens if these components fail. Test setups etc. are well-defined. You'll have to consider other failures too, e.g. what happens if the reduced clearance at your triac fails and a heat source that is intended to work on low duty cycle gets full power then? This can easily start a fire if no other countermeasures are provided.

That sounds fairly sensible but is it really necessary to perform tests?

Most potentially dangerous scenarios can be eliminated by good design. For example, an appropriately rated and approved thermal fuse, can be used to prevent a heater from overheating, if the TRIAC's junction fails short circuit, which is more likely than the functional insulation between the pins failing.

[capt bullshot]

That sounds fairly sensible but is it really necessary to perform tests?

Most potentially dangerous scenarios can be eliminated by good design. For example, an appropriately rated and approved thermal fuse, can be used to prevent a heater from overheating, if the TRIAC's junction fails short circuit, which is more likely than the functional insulation between the pins failing.

Well, that depends ...
Testing: You'd discuss this with your notified body or whoever certifies your product, differs with applicable standards.
Example: Yes, that's right, the TRIAC is more likely to fail than the clearance between its pins. Especially if you use TO220 cases, the wireframe has less distance than required for your PCB - that's kind of irritating and often solved by testing (TRIAC fails short includes isolation failure of the PCB).