Help with heater control

Actually..... with most heating systems...
The High Limit control, to shut off heat, cannot be only in the PLC... NFPA 86 (Standard for Ovens and Furnaces) regulates this.
You MUST have a separate relay contact from the High Limit that breaks the PLC output for the heat contactor.
You can still send the signal to the PLC for alarms and any other control.

Also the High Limit instrument MUST be "FM Approved"....or no insurance coverage.

In addition to fuses there exist circuit breakers with Z Characteristic Curve for Semi-Conductor Protection.

keithkyll said:

Getting back to the OP's question. There is two problems.
1) The contactor should have dropped out when the system overtemped from the shorted SSR. Why didn't it?
2) The SSRs are undersized.

Click to expand...

The contactor did not drop out because of poorly written PLC logic. Whoever wrote it put most of the soft overtemp cutoffs for the SSR coil, which didn't do any good once the SSR failed.

NetNathan said:

Actually..... with most heating systems...
The High Limit control, to shut off heat, cannot be only in the PLC... NFPA 86 (Standard for Ovens and Furnaces) regulates this.
You MUST have a separate relay contact from the High Limit that breaks the PLC output for the heat contactor.
You can still send the signal to the PLC for alarms and any other control.

Also the High Limit instrument MUST be "FM Approved"....or no insurance coverage.

Click to expand...

I was not aware of that at all, that's definitely important, thank you so much. Does that mean two contacts are needed, assuming one of them is controlled by the PLC?

lfe said:

In addition to fuses there exist circuit breakers with Z Characteristic Curve for Semi-Conductor Protection.

Click to expand...

So if I get a circuit breaker with Z curve would it be unnecessary to also get the fast blow fuses?



Also, the SSRs I've been seeing so far are pretty basic with just power going through and coil power to turn it on. I've found these SSRs with an alarm output and that's it so far. If I use an SSR without an alarm integrated in it is there some other way I can monitor it through the PLC to see if it has failed?

FactoryTalktotheHand said:

The absolute best option for heater controls is a Silicon-Controlled Rectifier or SCR paired with a shut-off Contactor.

Click to expand...

This is the way I was told to do it at one of my old jobs. I remember they wanted me to use Watlow SCR's. Watlow also recommended fast acting fuses that had to be sized properly. They had a pretty good user manual that explained which SCR to choose and which fuse size you needed.

Mousemania said:

So if I get a circuit breaker with Z curve would it be unnecessary to also get the fast blow fuses?

Also, the SSRs I've been seeing so far are pretty basic with just power going through and coil power to turn it on. I've found these SSRs with an alarm output and that's it so far. If I use an SSR without an alarm integrated in it is there some other way I can monitor it through the PLC to see if it has failed?

Click to expand...

Yes, the Z curve circuit breaker makes fuses unnecessary.

I used Carlo Gavazzi SSR before, good material, but I used simpler relays like this : https://www.gavazzionline.com/pdf/RGC_2_3A.pdf working with zero crossing switching and commanded by PLC transistor outputs then I had total control of the ON-OFF times.

As I said in some previous post I used Current Transformers to monitor resistances current with the corresponding alarms in case the measured current doesn't follow ON-OFF command orders. I know that this complicates a lot because many analog inputs are needed.

Monitoring transformers : https://www.gavazzionline.com/pdf/E832050eng.pdf

Avoid phase angle control, it generates a lot of electrical noise and sometimes audible noise too, it can make resonate the heating resistances, and transformers too, if any.

NetNathan said:

Actually..... with most heating systems...
The High Limit control, to shut off heat, cannot be only in the PLC... NFPA 86 (Standard for Ovens and Furnaces) regulates this.
You MUST have a separate relay contact from the High Limit that breaks the PLC output for the heat contactor.
You can still send the signal to the PLC for alarms and any other control.

Also the High Limit instrument MUST be "FM Approved"....or no insurance coverage.

Click to expand...

Mousemania said:

I was not aware of that at all, that's definitely important, thank you so much. Does that mean two contacts are needed, assuming one of them is controlled by the PLC?

Click to expand...

I use a relay on my High Limit.
I use one contact is to tell PLC the High Limit has tripped.
I use another other contact to break the signal from the PLC output to the Heat contactor.