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Old September 15th, 2020, 03:57 PM   #1
Jieve
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Protection for Single-Phase Immersion Heater

Hi all,


I have a single-phase (US, 240V, 6000W) on/off tank immersion heater that I need to decide on circuit protection for. I have been looking at overloads, 6000W = 25A. 3-Phase overloads are readily available, but as I understand it, due to the phase loss protection of the overload, I'd need to wire the return phase with the L2/T2 and L3/T3 connections in series. I've been looking at Siemens 3RV2 motor starter protectors for this, the datasheets specify a much lower rating when used with single phase for the same hp; I'm guessing this is because of the 1.732 factor between 3-phase and 1-phase power calculations. The model I'm looking at is rated to 7.5hp for single phase, meaning 23.28A, with adjustable thermal overload from 28-32A and is rated 15hp for 3-phase. I'm thinking trip class 10 is fine. These also have built-in instantaneous magnetic trip. So my questions:


1) Should I use any additional protection devices other than this? I was thinking of using a magnetic breaker ahead of this, but it seems unnecessary.


2) Should I worry about the fact that the single phase rating is a few amps below the heater current? The next step up for current range adjustment is significantly higher than the heater current.


3) What should the overload setting be set to, 1.25 x 25 = 32A approx? I see the 1.25 factor a lot, I think this is from NFPA79, is it applicable to resistance loads as well? Or should I set it lower?


4) I'm assuming that since this has a magnetic trip also, it should be installed upstream from the contactor, correct?


5) They have these in trip class 10 and 20, should I prefer one over the other? (my gut says go with 10)



Thanks!
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Old September 15th, 2020, 04:14 PM   #2
Robb B
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Quote:
Originally Posted by Jieve View Post
Hi all,


I have a single-phase (US, 240V, 6000W) on/off tank immersion heater that I need to decide on circuit protection for. I have been looking at overloads, 6000W = 25A. 3-Phase overloads are readily available, but as I understand it, due to the phase loss protection of the overload, I'd need to wire the return phase with the L2/T2 and L3/T3 connections in series. I've been looking at Siemens 3RV2 motor starter protectors for this, the datasheets specify a much lower rating when used with single phase for the same hp; I'm guessing this is because of the 1.732 factor between 3-phase and 1-phase power calculations. The model I'm looking at is rated to 7.5hp for single phase, meaning 23.28A, with adjustable thermal overload from 28-32A and is rated 15hp for 3-phase. I'm thinking trip class 10 is fine. These also have built-in instantaneous magnetic trip. So my questions:


1) Should I use any additional protection devices other than this? I was thinking of using a magnetic breaker ahead of this, but it seems unnecessary.



2) Should I worry about the fact that the single phase rating is a few amps below the heater current? The next step up for current range adjustment is significantly higher than the heater current.



3) What should the overload setting be set to, 1.25 x 25 = 32A approx? I see the 1.25 factor a lot, I think this is from NFPA79, is it applicable to resistance loads as well? Or should I set it lower?



4) I'm assuming that since this has a magnetic trip also, it should be installed upstream from the contactor, correct?



5) They have these in trip class 10 and 20, should I prefer one over the other? (my gut says go with 10)



Thanks!
1) What is feeding the Starter? Those wires need protection...

2) If the range of adjustment covers your heater, it's fine.

3) The 1.25x comes from motor ratings. Overloads are typically set to 1.25x the motors FLA rating, or per the Service Factor.

4) The starter is a contactor with overload detection...

5) Trip classes have to do with time duration and loads.

You should leave the installation of the Heater and protection of it, to a trained electrician. I'm 100% certain the documentation for the heater, starter, whatever will confirm my statement. There's a lot that could go wrong. At a minimum, read the documents very carefully, and consult the manufacturer as to the suitability of the device for the job.
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Old September 15th, 2020, 04:32 PM   #3
Jieve
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1) I'm only describing the heater circuit. The mains above that will be properly protected.


4) The device is a starter protector, not a starter. No contactor included.


5) Yes, I'm aware of this. The question is whether or not Class 10 vs. Class 20 would be appropriate for this application.


Looking at the NFPA79 section 7.2.11 regarding overloads for resistance heating, it only says for resistance heating loads 16.7A or more, overload should be set not higher than 150%. For 48+A, setting should be not greater than 125%. I guess that answers the question on overload setting.



It doesn't mention anything about trip class though, so thought maybe someone else could weigh in on this.
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Old September 15th, 2020, 05:00 PM   #4
James Mcquade
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class 10 means that the overload will trip after 10 seconds of overload current.
class 20 means 20 seconds before tripping.
class 30 means 30 seconds before tripping.
i'm not clear about your setup but you will need the following.
from the main disconnect, you will need fusing before the contactor to protect the wiring, a contactor, then the overloads to protect the wiring and heater element. i would call your local allen bradley supplier for example and discuss your application.
james
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Old September 15th, 2020, 05:07 PM   #5
g.mccormick
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I'm confused on why you want to use an overload for a resistance heater? The heater circuit should just need fusing/circuit breaker. I would do CB -> overtemp safety contactor -> SCR/SSR -> Heater
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Old September 15th, 2020, 05:28 PM   #6
padees
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Quote:
Originally Posted by g.mccormick View Post
i'm confused on why you want to use an overload for a resistance heater? The heater circuit should just need fusing/circuit breaker. I would do cb -> overtemp safety contactor -> scr/ssr -> heater

^^^yep^^^
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Old September 16th, 2020, 10:50 AM   #7
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+2. Overloads are not used for heaters in my experience. Fast acting fuses are my preference since there is no inrush to worry about, and if the current exceeds the fuse rating, something has gone very wrong and needs to be shut off fast.
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Old September 16th, 2020, 03:32 PM   #8
Jieve
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Quote:
Originally Posted by g.mccormick View Post
I'm confused on why you want to use an overload for a resistance heater? The heater circuit should just need fusing/circuit breaker. I would do CB -> overtemp safety contactor -> SCR/SSR -> Heater

Thanks for this feedback. By overtemp safety contactor, do you mean a safety relay/contactor monitoring an additional RTD/Thermocouple?


My original reasoning was that there are two things (electrically) to be concerned about, short circuit and overload, and as rupej mentioned, since inrush isn't a concern and small overload could mean bad news, I'd have more control over the overload current with a motor starter protector, and it already has a magnetic CB built-in. The fast reaction time was why I was thinking a class 10 overload.
But I'd definitely prefer to do the way you suggest, if my approach isn't common.


This is just a small system in a non-production environment where there isn't a massive risk if things aren't done in a specific way. But again, I'd prefer to do it correctly, which is why I posted here for feedback.



The SSR here would be used to facilitate switching PID/Temp control, correct?

Last edited by Jieve; September 16th, 2020 at 03:35 PM.
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Old September 16th, 2020, 04:30 PM   #9
AkaHammer
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what g. mccormick said. If you want safe you fuses they trip way faster than circuit breaker and overload is a thing for motors not heater so you don't need overload only short circuit protection
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Old September 16th, 2020, 05:56 PM   #10
g.mccormick
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A resistance heater does not have overload or inrush. If it is a 240VAC and 6000W, then the current will be : P=V*I -> I=P/V = 6000/240 = 25A. I would through a 30A breaker or fuses in.

Does the heater have built in thermostat or thermocouple? The safety contactor would be used to interrupt the power to the heater/SSR in an event of overheat. SSRs canand do fail ON. The contactor ahead of it will disable heat. Typically you would also disable the contactor for when heat circuit is not needed. What is your control means?
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Old September 16th, 2020, 06:02 PM   #11
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Here is a very quick from google search. You may likely have the application would the B contactor coil turned on by a secondary safety means.
Attached Images
File Type: png Three_Phase_Supply_Modulating_Pneumatic_Signal.png (113.0 KB, 32 views)
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Old September 16th, 2020, 06:13 PM   #12
Jieve
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Quote:
Originally Posted by g.mccormick View Post
A resistance heater does not have overload or inrush. If it is a 240VAC and 6000W, then the current will be : P=V*I -> I=P/V = 6000/240 = 25A. I would through a 30A breaker or fuses in.

Will do.


Quote:
Originally Posted by g.mccormick View Post
Does the heater have built in thermostat or thermocouple? The safety contactor would be used to interrupt the power to the heater/SSR in an event of overheat. SSRs canand do fail ON. The contactor ahead of it will disable heat. Typically you would also disable the contactor for when heat circuit is not needed. What is your control means?

No built-in thermostat or thermocouple. Using a PT100 with 4-20mA transmitter for temp feedback into a PLC. On/Off was planned to be controlled by the PLC through a contactor.



I found a couple other references suggesting using an SSR as opposed to contactor for heater control, so found a 30A solid-state contactor that should do the trick.


I'm not entirely familiar with safety controls on process systems, but quite familiar with them for machine guarding. My assumption based on what I've read in other posts is that the safety on this should be independent of the control, meaning a second PT100 fed into a temp monitoring relay that controls a contactor in the main circuit. Nothing I've seen of these components mention anything about safety ratings ... would it make sense to run the temp monitoring relay outputs to a safety relay, which controls the contactor?
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Old September 16th, 2020, 09:03 PM   #13
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When running a heater with SSR, I would recommend using a 2 pole shunt trip breaker upstream of it. Solid state relays frequently fail shorted and can cause the temperature to run away, so if you use a shunt trip breaker you can wire an output to trip it if the temp sensor faults or gets out of range. The SSR will outlast a contactor as long as it doesn't get overheated and is properly protected, but chit happens, usually to the field wiring, so when something does try to let the smoke out, you can hopefully save your heater from catastrophic failure with the shunt trip added to your breaker.
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Last edited by OkiePC; September 16th, 2020 at 09:07 PM.
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Old September 16th, 2020, 11:11 PM   #14
James Mcquade
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i'm with Okiepc on this one. i would use a 40 amp 2 pole mercury contactor rather than an scr/ssr. they do short out and inexperienced electricians will assume a relay / contactor and will end up getting shocked.
james
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Old September 16th, 2020, 11:34 PM   #15
Jieve
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Quote:
Originally Posted by OkiePC View Post
When running a heater with SSR, I would recommend using a 2 pole shunt trip breaker upstream of it. Solid state relays frequently fail shorted and can cause the temperature to run away, so if you use a shunt trip breaker you can wire an output to trip it if the temp sensor faults or gets out of range. The SSR will outlast a contactor as long as it doesn't get overheated and is properly protected, but chit happens, usually to the field wiring, so when something does try to let the smoke out, you can hopefully save your heater from catastrophic failure with the shunt trip added to your breaker.

What would be the benefit of using a shunt breaker as opposed to fast acting fuses and a contactor? I could just as easily wire up an output to open a, say, 40A contactor ahead of the SSR, with fuses ahead of that.


Is no one on board with my temp monitoring relay and second sensor idea? Unnecessary? I had actually downloaded an example of this for furnace control from this site a few months back. See image attached
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File Type: gif Furnace Control w Safety.gif (25.2 KB, 22 views)
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