Current rating's of switches, is my math right?

[Vinylwasp]

Hey guys, when I built my brewery I used some switches that are advertised as 16A 250V/20A 125V which I assumed would be sufficient for my build using 2500w elements.

As you can see below, the switch failed in the middle of my last brew which I ended up replacing during the boil with a similar switch with the same rating. It had clearly got really hot and melted the tiny piece of plastic that held the pole into the switch and it just fell out breaking the circuit.You can see that the other one was starting to go the same way. Not great.

Replacement on right. (same rating but looks better made).

I'd been feeling a little uncomfortable about the wisdom of using switches with the same 16A 250V/20A 125V rating that had already failed, so I replaced them both with two of these 20A 250V switches last week.

So here's my question. If a switch is rated at 20A 125V surely it's 250v rating is 10A, not 16A or is there something about resistance here that I don't understand?

 

[jof]

I think it's more complex than that.
You have arcing on the switch contacts as well as the resistance of the metal parts that carry the current.

Heating generally occurs due to resistance, so the thinner the copper, the more resistance & more heat generated.
That is further complicated by the area of contact between the switch terminals and the wire (be that the crimp connector, screw terminal or even soldered joint) and if the terminal has got tarnished.

Always best to over rate the switches & cables, especially as your brewing equipment is likely to be left on for long periods.
Eg:My domestic kettle lead starts to warm up nicely if you boil 3 full loads in rapid succession.

 

[Buffers brewery]

Like you, I would’ve assumed a switch rated at 16 A @ 250 V should be able to handle 2500 watts. So two questions. 1. Was the switched line feeding other equipment in addition to the element? 2. Did you purchase the switch from a reputable electrical equipment supplier (RS, Farnell)? If the answer to both questions is “No” I can only think it’s a “translation” problem, Chinese to English!

 

[MashBag]

"translation" is a lovely phrase. I will not mention "cheap Chinese sh1t3" again. Which is most often the issue.

I have some "4000w power controllers" ... Not on chance 500w max imo.

I am having the same problem with iec c15 hot condition "kettle" plugs. 10 amp... Really.... These even look sh1te in the picture FFS.

 

[Vinylwasp]

I think it's more complex than that.
You have arcing on the switch contacts as well as the resistance of the metal parts that carry the current.

Heating generally occurs due to resistance, so the thinner the copper, the more resistance & more heat generated.
That is further complicated by the area of contact between the switch terminals and the wire (be that the crimp connector, screw terminal or even soldered joint) and if the terminal has got tarnished.

Always best to over rate the switches & cables, especially as your brewing equipment is likely to be left on for long periods.
Eg:My domestic kettle lead starts to warm up nicely if you boil 3 full loads in rapid succession.

When those two switches are turned on, there's zero load until the Solid State Relay's kick in which control power to the elements. There's also a brewing pump powered through the same arrangement with a separate SSR in each cabinet, but the total shouldn't be drawing above 13A.

You can see in the picture of the failed switch that the contact area is very small (maybe 2.5mm round) which I think is way under-engineered to handle 16A @250V, and you can see that it's heated up, melted the tiny piece of plastic that holds it in the switch body, and simply fell out. All the cables (4mm) and plug contacts were fine.

 

[Vinylwasp]

Did you purchase the switch from a reputable electrical equipment supplier (RS, Farnell)? If the answer to both questions is “No” I can only think it’s a “translation” problem, Chinese to English!

Amazon.
When I did my apprenticeship, the pricing manager had this sign hanging up over his door. It's guided me well until now, but it's so hard to get stuff now. There used to be a Maplins in town but it's long gone and RS prices are insane.

 

[Buffers brewery]

Amazon.
When I did my apprenticeship, the pricing manager had this sign hanging up over his door. It's guided me well until now, but it's so hard to get stuff now. There used to be a Maplins in town but it's long gone and RS prices are insane.

View attachment 96337

https://cpc.farnell.com/molveno/sx8...MImtinicLThAMVtptQBh1yIAjOEAQYBSABEgKliPD_BwE

 

[Vinylwasp]

https://cpc.farnell.com/molveno/sx8211881g110000/rocker-switch-20a-red-dp-off-on/dp/SW05303?mckv=s_dc|pcrid|605262956782|kword||match||plid||slid||product|SW05303|pgrid|138313686895|ptaid|pla-2301374426797|&CMP=KNC-GUK-CPC-SHOPPING-9262013734-138313686895-SW05303&s_kwcid=AL!5616!3!605262956782!!!network}!2301374426797!&gad_source=1&gclid=EAIaIQobChMImtinicLThAMVtptQBh1yIAjOEAQYBSABEgKliPD_BwE

Yep, good price, but you have to pay for shipping and this looks to be the same as the temporary replacement switch I fitted.
Note the different ratings from the spec sheet. I have no idea why these are different (10, 16 + 20A @ 250 VAC) so I think I'm going to continue to add a healthy amount of overcapacity to any work I do in the future. See the inside of the 20A switches I fitted, they're at least 10x the material of the one that failed.

 

[Vinylwasp]

Updated my blog with an entry on fixing this switch burnout issue.
After 1 brew it seems to have solved the problem.

 

[peterpiper]

So here's my question. If a switch is rated at 20A 125V surely it's 250v rating is 10A, not 16A or is there something about resistance here that I don't understand?

If the switch was already in the on position, and then you plugged in some load. It would handle 20A, no matter if the supply was 125V or 250V. As it's the switch resistance and current, that governs heat generated, which is the limiting factor.

It's the arcing, that largely happens when the is switch turned off while under load, that leads to the different current ratings.
At 250V the arcing will be more severe, and likely to causie more contact damage, than at 125V.
.
After any arcing, the resistance can increase. Its this higher resistance that causes overheating, and burn out.
Use of a suitable switch contact material, reduces damage from arcing. Contacts should be mase of semi-refractory materials (mostly silver compounds), for this type of switch.

Arcing, as a switch with oxidised or dirty contacts is closed, can form a micro spot weld(s) between the contacts. This can make a much better contact, than the touching faces alone would.

Strong switch contact pressure, or good contact action (eg sliding contact or rapid action) is better at cleaning the contacts (of any arcing deposits) as the switch is operated.
To help in keeping the the contacts clean, operate the switch without any load (unplugged), half a dozen times.

2500W on a 230V supply, is around 11A

When a (new working) switch is closed, the contact resistance should ideally be 0 Ω (Ohms).
Say the switch in question was initially 0.01 Ω

Power (Watts) = (current (Amps) )squared * resistance (Ω)
Power (dissapated in switch) = 11*11 * 0.01 = 1.21 W (switch might feel warm)

After arcing damage, a contact restistance of 0.3Ω is quite likely
Power (dissapated in switch) = 11*11 * 0.3 = 36W (switch will melt).

There are many bad switches, that should not be on sale in the UK.
Out of the first five Toolstation cheap brand, 13A switched double sockets (fitted during a house rewire). I found three had faulty switches, that failed the EIRC testing. Having variable contact resistance up to 0.3 Ω. So the whole lot went back.
I now always use reliable MK brand ones, made in UK.