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PCB trace used as a fuse?

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There is a reason why fuses cost as much as they do:

1) Their fusing current vs time (what is called I-squared-T curves) is well defined, repeatable across the population and consistent, even with temperature changes.

2) Their clearing current capacity...don't confuse the fusing current with clearing current. Once that a fuse opens, an arc will form. If your power source has enough energy capability, this arc will not dissipate on its own, but it has to be quenched. True, give it enough time and it will quench on its own, but not after having burned something up.

As I explained, my personal experience was with a cheap Chinese CFL. It cost me US$1 each back in the late 1990s which was a bargain.

I was working on my computer at home, when something shorted inside the CFL. A loud bang, and lots of acrid smoke started coming out from the light fixture. Fortunately, I was there and knew exactly what to do, turn off the light switch.
Had the room been alone or with my wife there, she would just have panicked and ran.

When I performed an autopsy on the unit, I found the brown paper phenolic PCB, which I don't think meets UL94-V flammability ratings, had an almost 1 cm wide burnt area.

I removed and threw away all the lamps I had purchased.
 

  • smoking epoxy is known to be carcinogenic with a soldering iron. (how much I dont know)
  • copper melting point is far above the temperature of a soldering iron.
  • FR4 is made from epoxy and fibreglass weave.
  • If a trace gets hot enough below the copper melting point, the epoxy will smoke
  • Therefore designing a fuse on FR4 is a bad idea from a design point of view.

p.s. An SMD fuse chip is OK as it has been designed to be safe and accurate. These are often found on power inputs on HDD cct. boards.
 

schmitt trigger said:
There is a reason why fuses cost as much as they do:

1) Their fusing current vs time (what is called I-squared-T curves) is well defined, repeatable across the population and consistent, even with temperature changes.

2) Their clearing current capacity...don't confuse the fusing current with clearing current. Once that a fuse opens, an arc will form. If your power source has enough energy capability, this arc will not dissipate on its own, but it has to be quenched. True, give it enough time and it will quench on its own, but not after having burned something up.

As I explained, my personal experience was with a cheap Chinese CFL. It cost me US$1 each back in the late 1990s which was a bargain.

I was working on my computer at home, when something shorted inside the CFL. A loud bang, and lots of acrid smoke started coming out from the light fixture. Fortunately, I was there and knew exactly what to do, turn off the light switch.
Had the room been alone or with my wife there, she would just have panicked and ran.

When I performed an autopsy on the unit, I found the brown paper phenolic PCB, which I don't think meets UL94-V flammability ratings, had an almost 1 cm wide burnt area.

I removed and threw away all the lamps I had purchased.
Click to expand...

If you want my opinion on why fuses cost as much as they do, the one word answer would be *greed*.

About your point on the arcing, that is a nagging problem with all power switches. Domestic switches work well on AC because the arc will anyway go out in 1/4 cycle. If you use the same switch on your circuits on the DC side, they will last far less. Many cartridge fuses are equipped with a spring that extinguishes the arc. Total amount of energy the fuse can dissipate depends only on the design of the fuse and not on the 'energy capability of the power source'- circuit breakers that control MW of power have progressively become smaller and smaller.

Many branded (very respectable American and European) TV brands (CRT type, older models) I have seen used paper phenolic PCB boards.
 

Which particular standards say that a PCB trace cannot be used as a fuse ??

Standard no, section no please?
 

Smoke and flammability stds, vary in each country. pick any

what temperature is the copper at max load test?
dielectric Glass transition temp? 200'C?
max temp without fusing? 900'C
Copper melting point. >1000'C
Ratio of fuse current/load current?

Will it smoke under any conditions? will it cause toxic fumes?
 

Any country will do, Europe or US ?
I'm not wanting to read through a whole pile of standards till I find one that mayor may not have it in.

If you have a board that takes milliamps to run and its designed such that should there be a catastrophic fault causing a short on the power the main power track will also burn out - which is unlikely to set the whole board on fire (no fuse at all is likely to do that) then Id like to understand the exact requirement that prevents this rather than just "I'd not do that myself" type opinions.
 

Saturn PCB toolkit has a fusing current calculator...
Read the article, copper resistivitity can vary by 40%, add manufacturing tolerances, then look at your layer stack and tell me if you can design a reliable fuse, why push an idea that is dangerous and these days safety is paramount. The problem is if something does go wrong then liability comes up, one perfect reason to do the job properly.

- - - Updated - - -

Standards for fuses...
https://en.wikipedia.org/wiki/IEC_60269
 

The calculator must only be used to to estimate the short circuit current that will vaporize a track and NOT to protect a circuit or system or distribution wire from damage or fire.
 

marce said:
In over 30 years I have never seen a PCB trace used as a fuse, I would never do it and all designers I know would never do it...
Basic thoughts, a fuse is deigned to do a job, they are tested and certified, a PCB trace used as a fuse is not tested so you are relying on an untested and unreliable PCB trace to fuse to prevent damage due to some high current incident...
Not a good idea.
Click to expand...

Yep I've been saying this all along...:)
 
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schmitt trigger said:
I've seen traces used as fuses, but only in cheap, throwaway Chinese products.
In the particular instance, it was an inexpensive CFL.

The only reason not to use a fuse is because you intend the product to be disposable. And save a few pennies.
Click to expand...

chinese products are not throwaway, hey, Everything has it's price, of course, have the cheap one , the expensive one.
 

Sinoble, please explain your post.....
The reference was to a cheaply manufactured product in this case a CFL, that are designed to be thrown away after use, a one off product with a limited life span, its a light bulb....
Soo as I said please explain your post in more detail as I do not understand your umbrage towards the post quoted, not do i understand the relevance of your post to this discussion.
 

I think the assumption in question is cheap means non repairable, or at least not easily repaired by a novice, but maybe repairable by some.

In this case a PCB fuse is not... hence a cheap solution to make it non flammable. even if using FR4 ( epoxy fire retardant level 4) at least less toxic carcinogenic fumes.
 

Fuses are sacrificable devices (there are resettable polymer fuses though) that are expected to commit suicide just to save some other parts, usually damage from a higher current or voltage. In that philosophy, any fuse is better than no fuse. If a PCB trace used as a fuse can save us from a bigger fire hazard, then it must be welcome (notwithstanding standards).

Modern designs tend to self-limit (current) and hence fuses are slowly becoming redundant- less and less instruments now depend on fuses for protection.
 

c_mitra said:
Modern designs tend to self-limit (current) and hence fuses are slowly becoming redundant- less and less instruments now depend on fuses for protection.
Click to expand...
I would be very cautious of this statement. It may well be true if the circuit is still working to some extent that it can limit the current itself draws, but there are many failure scenarios that are external to the circuit (for a silly example: someone sticking a screwdriver across the power regulator input terminals) where the current can be way above the designed maximum.
To my mind, fuses in power leads are a cheap an deasy way to protect against all sorts of problems - hoverboard fires for example.
Susan
 

Modern designs tend to self-limit (current) and hence fuses are slowly becoming redundant- less and less instruments now depend on fuses for protection.
Click to expand...

Fuses are required by safety standards in many circuit situations, particularly UL and CS are very strict in avoiding fire hazards. Simple "self-limiting" means without redundancy are hardly accepted by the regulations. Simply because semiconductor components are assumed to fail potentially.

As a result you have a set of fuses that are required for short circuit and possibly overcurrent protection. Fuses may be omitted if short circuit is sufficiently handled by an upstream fuse, e.g. the mandatory 10 or 16 A fuse of a wall socket and overload doesn't risk fire or personal injuries.

There's also a range of optional fuses intended to avoid component damage in case of overload. In this field, electronic protection can be applied beneficially. Or a planned obsolescence strategy decide to economize it.
 

Mattylad said:
Which particular standards say that a PCB trace cannot be used as a fuse ??

Standard no, section no please?
Click to expand...

Safety standards are deep documents which require specialists individuals to follow.
I'm no such a specialist, but in the company which I work for, we do have such a group of specialists in the corporate offices.

Products which have their power supply integrated into the main board, have to have a protective feature of last resort. Which cannot be a fusible trace.

- - - Updated - - -

c_mitra said:
Many branded (very respectable American and European) TV brands (CRT type, older models) I have seen used paper phenolic PCB boards.
Click to expand...

I know, I've personally seen that.

But we are discussing the merits (or lack of) a fusible trace.
 

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