Current rating of Faston / Lucar connectors

HI All I'm thinking of replacing the main relays in my 7.5kw glass-fusing kiln with these relays https://ie.farnell.com/potter-brumfield-te-connectivity/t92p11d22-12/relay-dpdt-277vac-28vdc-40a/dp/270349#anchorTechnicalDOCS
- which are Potter&Brumfield DPDT 40A relays.
Currently I'm using domestic contactors, which last a few years (not through being overloaded, simply through the number of on/off cycles they go through during each firing schedule). The contactors have screw-down clamps for the connections.
The Potter&Brumfield relay uses 1/4" 'Faston / Lucar' connectors. Out of curiosity, I looked on the web for the current rating of these connectors - Wiki claims "24A (continuous)" with 10AWG wire (about 5.2mm-squared).
Any thoughts on this - or how a relay rated at 40A has connectors rated at 24A?
My plan was to find the sturdiest Lucars I can locate, crimp them onto the mains feeds and (?perhaps?) run some solder into the crimp - just to be sure.
What does the team think? Thanks
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Adrian Brentnall has brought this to us :

Why not look at converting over from relays, to triac switching?
https://uk.rs-online.com/web/c/semiconductors/discrete-semiconductors/triacs/?cm_mmc=UK-PPC-DS3A-_-google-_-1_UK_EN_G_Semiconductors_Discrete%20Semiconductors_BMM_%7BAS_Test%7D-_-TRIACs-_-%2Btriacs&matchtype=b&kwd-25311081626&s_kwcid=AL !7457!3!377872242924!b!!s!!%2Btriacs&gclidIaIQobChMImI7fv4bQ5QIV2YjVCh3KDQDVEAAYASAAEgKcOvD_BwE&gclsrc=aw.ds
BTA40-600B is rated at 40amps 600v
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On 04/11/2019 07:40, Harry Bloomfield wrote:

HI Harry. I've looked into the whole 'Solid State Relays' thing, and, to be honest, I'm not convinced.. When you examine the specs, these things need some serious heatsinking (as they have a finite 'on' resistance) and, call me old-fashioned, but I rather like to hear the relay clicking on & off, as it reassures me that all is running properly. Retro-fitting the SSR plus the heatsink and probably a small cooling fan would be difficult in the existing enclosure.
The big worry with glass-fusing kilns is that the main relay fill fail 'on' (welded contacts) - as when this happens the kiln will continue to heat uncontrollably until something melts or catches fire.
Seems that the solid-state 'switches' can fail 'on' or 'off' - just like the electromechanical relays. I know that they have a longer predicted life - but the high-powered ones are also several times more expensive than the equivalent relay.
I think relays are the way to go - and to treat them as a 'service item' and swap them out every three years or so (based on the predicted cycle life).
I was more concerned about the best way to terminate the wiring - given that the Faston connectors are apparently rated at 24A and the kiln's pulling 30A or so while the relay's rated 40A... Maybe it's a non-problem, and Faston are just being cautious?
Thanks
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On Monday, 4 November 2019 08:00:09 UTC, Adrian Brentnall wrote:

riacs/?cm_mmc=UK-PPC-DS3A-_-google-_-1_UK_EN_G_Semiconductors_Discrete%20 Semiconductors_BMM_%7BAS_Test%7D-_-TRIACs-_-%2Btriacs&matchtype=b&kwd-253 11081626&s_kwcid=AL!7457!3!377872242924!b!!s!!%2Btriacs&gclidIaIQobC hMImI7fv4bQ5QIV2YjVCh3KDQDVEAAYASAAEgKcOvD_BwE&gclsrc=aw.ds

The relay is two-pole, so why not parallel the two poles and share the current across two sets of Faston connectors? John
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On 04/11/2019 08:29, snipped-for-privacy@gmail.com wrote:

That might be a plan.. In fact, the 7.5kw is actually made up of two separately-wired heating elements - with two cables running back to the control box - so switching each one through its own set of contacts would reduce the current on each Faston. The contacts themselves would probably appreciate it too...
The currently-installed contactors are also double-pole - and I was switching both Live and Neutral - but I guess there's no real need to do so...
OK - thanks!
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Adrian Brentnall used his keyboard to write :

If I'm reading the datasheet correctly, it suggests 1 watt dissipation. It will be smaller than you contactor. You could always add an indicator light and even a small relay just to make the reassuring noise.
I've had such a SSS switching in my garage every few seconds, for the past 20 years. Zero switching, they generate no switching surges.
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On 04/11/2019 08:30, Harry Bloomfield wrote:

I did the sums a while back, and I seem to recall it was nearer 60 watts.. and the warmer you run them the more you have to derate them
The SSRs I was looking at were the same form factor as the contactors (DIN rail mounting), before you added the heatsink.
Zero-switching is about the only advantage I could see in using SSRs...
I think I'll stick with good old relays! (Fully-paid-up Luddite, me!)
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On Mon, 4 Nov 2019 08:00:06 +0000, Adrian Brentnall

I have similar reassurance on the (12V / 120W) heated bed of my 3D printer as that's powered (Bang-bang) via a HD automotive relay.
The extruder is electronic (PWM) driven by the main unit but is rarely on continuously, only during initial heatup (probably 60 seconds or so).
If you tell it to preheat, you see the outputs for both the extruder and bed via their LEDs (and there is an LED on the bed itself) but it's the click of the bed relay that you hear when you aren't looking at the printer. Ok, when the printer is actually printing you hear that as well but once running I'm less bothered than during the startup.
I believe I did try a SSR on the bed but I also think I remember it getting very hot but I can't remember if I was using it BB or PWM (although I'm not sure that would have mattered during warmup as it would have been on continuously in any case).
I think the idea of splitting your load of one heater per connector on a dual pole relay or how about one heater per relay (again, just switching the lives, assuming they aren't that expensive and the input current for the coils low)?
I think you wire contacts in series if you want them to last longer (less arcing during disconnection) and you shouldn't parallel contacts to the same load, other than for redundancy and assuming one contact can still carry the required load.
Cheers, T i m
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On 04/11/2019 10:05, T i m wrote:

In glass-fusing it's all about a series of segments - each one has a ramp speed, a target temperature and a hold time - up to 7 or 8 per schedule - so the relay is worked fairly hard by the controller as it turns the power on & off to achieve the desired profile...
I had thought about running two relays, one per heating coil - but I'm not sure that achieves much advantage over running the two could from two poles of the same relay.. Sooner or later, the switching contact will fail - probably in the 'on' position (ref Murphy's Law!)
Running the two poles to the two heaters gets over the slight worry about the current-handling capacity of the Fastons...
Thanks
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On Mon, 4 Nov 2019 12:11:13 +0000, Adrian Brentnall

Other than the load is then carried over two mechanically (thermally) different modules, something that could be of value if the temperature rise de to connector and contact resistance may be concerned?

I think the normal reason for contact welding is a high resistance built up from a mixture of contact bounce on connection and arching on disconnection. I think the suggestion of running two contacts in series is supposed to help alleviate that.

And even better to have them on two complete different relays (and only because your load is naturally formed by two separate devices). ;-)
Cheers, T i m
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On 04/11/2019 13:10, T i m wrote:

Yes - I wasn't sure whether 2 poles or 2 relays was the way to go...
As I say, the plan is for these relays to be 'service replacement' items - so it's really about making sure they run happily for the 3 years or so between replacement...
Thanks
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On Mon, 4 Nov 2019 12:11:13 +0000, Adrian Brentnall

I am fascinated by all those PID's (are they called) and I believe my printer firmware (Marlin) has the ability to create the PIDs itself by running the heaters up and measuring the response and noting any overshoot etc. Once done, that becomes the working profile. ;-)

As an aside, we know someone who used to work in a 'Glassworks' and it is interesting hearing about all the process and what they had to do when things went wrong.
Cheers, T i m
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On 04/11/2019 13:42, T i m wrote:

Yes - that's how it works. At frequent intervals the micro looks at the 'travelling setpoint' - which is where it thinks the temperature _ought_ to be, and tweaks things up or down to compensate. There's a 'learning' process running all the time...
This one is fairly easy on the controller as the kiln is lined with ceramic fibre - so there is very little thermal mass to consider - unlike a brick-based kiln where a lot of the heat goes into warming the bricks up.

Yes - can get exciting.. As with all these situations - it's about doing the "what if's" - and having a contingency plan!
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On 04/11/2019 13:42, T i m wrote:

P gives proportional control, giving higher control effort the further the measured value is away from the setpoint. Moving rapidly towards the setpoint but ending up with some overshoot and final offset.
I gives integral control, higher control effort the longer the measured value is away from the setpoint. Closing the offset to give a zero error.
D gives derivative control, reacting to rate of change of the error to allow maximum rate of change, while reducing it at the last moment to reduce overshoot.
Switching on and measuring the rate of change, overshoot and offset allows a simple calculation of the figures to enter into the controller for the best response. Autotune just does the calculation in the controller and stores the figures itself, cutting out the human part of it.

I've walked around a bottle making plant, seeing slugs of semi-molten glass drop through holes in the ceiling, be deflected by strategically located channels and fire off through other holes to the next part of the process.
SteveW
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On 06/11/2019 01:01, Steve Walker wrote:

Having worked on industrial control systems - i can't watch those sort of factories on tv without thinking to myself 'what happens when it all goes wrong...?'
It's bad enough in the food factories, where all they're making is biscuits or chocolates, but when you've got molten glass plying about the place, it could all get much more exciting much more quickly..
They never show you that bit!
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On Wed, 6 Nov 2019 01:01:41 +0000, Steve Walker

Thanks for that.
I think I'm running an older version of Marlin in my Mendlemax 3D printer but loath to change it because it works (and often like the 'tool' I always hoped it to be, after the project / hobby it initially was).
If you could pull the existing firmware out of an Arduino Mega I might be more keen to give the latest one a try but I don't think you can.
I think I have a complete set of spare electronics so might set that up with the latest firmware and swap the current one out as a test.

That sounds like a good idea, as long as it works etc.

I think I can trump that round a steelworks (whilst looking at 'Heavy Industry' with a college group for a week) and that looks and *is* very dangerous (or maybe was more dangerous then).
Second to that was probably a Lead Acid battery factory and a chroming plant. ;-)
Cheers, T i m
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On Wednesday, 6 November 2019 10:48:15 UTC, T i m wrote:

How about a copper refinery where I had to walk along an overhead steel gantry with rather feeble looking railings over a huge open crucible of molten copper.
John
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On 04/11/2019 08:00, Adrian Brentnall wrote:

Use two force-guided relays in series and use the NC contacts for monitoring? If one relay welds shut, the other should still cut the power and you'll get a warning that one has failed.
SteveW
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On 06/11/2019 00:47, Steve Walker wrote:

Hi Steve Not sure about 'force-guided' - not a term I know?
The idea of the safety relay is that is is in series with the 'main' relay - but the safety relay is only switched once per firing schedule, while the 'main' relay can be cycled several thousand times per schedule.
On that basis, the 'safety' relay should wear out less fast, and should be reliable enough that it can be expected to interrupt the power when/if asked to do so.
If I was designing the controller from scratch I would want to see a pair of 'signal' contacts on each relay - so I could do the usual - ask for the relay to close - wait a bit (in computer terms... to allow the contacts to close and stop bouncing) - check the signal contacts to confirm that it's closed - throw an error / shut things down if it hasn't
and vice-versa when asking the relay to open.
But, in this case, if I can allow the controller to drop out the 'safety' relay when it feels uneasy about what's occuring, and, possibly, use the remote monitoring Raspberry Pi as a second sanity check (by watching the kiln temperature with a second thermocouple and breaking the 'safety' chain if it goes above expected limits), then I reckon it'll be a vast improvement over what's there already.
Thanks
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On 06/11/2019 08:08, Adrian Brentnall wrote:

Force-guided relays have NO and NC contacts that are physically connected, so if a NO contact welds together, the NC contact cannot close, thus allowing allowing failure to be detected.

Perfectly reasonable.

That's where the force-guided relay comes in - allowing the NC contacts to be running at control voltages rather than power voltages, but showing when there is a fault.

Sounds like a reasonable plan.
SteveW
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