Branch circuit protection and I^2T fuses on SSR-driven heater

[LowEnergyParticle]

Hello all,<br><br>I'm a new member here, and have read through a number of posts in the forum.  There are a lot of interesting topics and some very skilled and experienced people here!  I'm looking forward to learning a lot.  <br><br>I've got a little government-work after-hours project going on that I need some advice and clarification on.  My brother lives in the Frozen North, and has a small machine shop at his home in an enclosed but unheated room.  He's interested in keeping the machines from rusting, but is a notable cheapskate (particularly when it comes to paying the electric bill).  I've designed a small control panel that runs a 120 VAC oil-filled 1.5 KW (12.5 amp) heating element based on a calculation of ambient room temperature, machine steel temperature, and relative humidity.  The business-end of the of panel is just a single 25 amp SSR.  Since SSRs tend to fail shorted, I've programmed a high-temperature switch to open a contactor just upstream of the SSR: if the room achieves a temperature 15 degrees F above the setpoint, the contactor will open and drop the heater.  The contactor will not re-energize until the high temperature condition clears and a reset push button is pressed.  <br><br>Here's an ASCII version of the power one-line:<br><ol><li>20 amp single-pole breaker in the panelboard<br></li><li>20 amp receptacle adjacent to panelboard.  receptacle is 8 feet from control panel, and in line-of-sight.<br></li><li>20 amp plug wired to 2 feet of 12 AWG SOJ cable, spliced in utility box to 3, 12 foot lengths of 12 AWG THHN in conduit between panelboard and control panel.  <br></li><li>In the control panel, both the hot and neutral conductors land on the contactor (opens on high temperature)<br></li><li>12 AWG neutral conductor wired from contactor to simplex receptacle mounted on bottom of control panel<br></li><li>12 AWG hot conductor wired from contactor to output-side #1 of SSR<br></li><li>12 AWG hot conductor wired from output-side #2 of SSR to simplex receptacle mounted on bottom of control panel<br></li><li>!20 VAC oil-filled 1.5 KW (12.5 amp) heating element comes provided with a 2-conductor plug: plugged into simplex receptacle.  <br></li></ol>Here are my questions:<br><br>A.  Is the 20 amp breaker in the panelboard (item 1 above) sufficient for branch circuit protection, or must the control box contain another breaker?  The control box never branches again, there's only the 1 circuit, and the 20 amp breaker at the panelboard is sized correctly.  <br><br>B.  The SSR manufacturer wants a fast I^2T fuse on the output of the SSR.  The fuse and block cost quite a bit more than the little 25 amp SSR.  The heater will not have the 6x startup surge of a motor, and it also doesn't have to respond to load surges like a motor, so I'm kind of thinking that I should keep a replacement SSR around and not install the I^2T fuse at all.  Is this a violation?<br><br>C)  The transceivers and logic elements in the control loop are all mounted in the control panel and powered at 120 VAC.  I want to provide a 1 amp AG slow blow fuse to power the equipment, and have the fuse powered by jumping from the hot-side of the contactor in Step #4 above.  This seems fine from an ampacity view point, but I'm not sure if I've created a branching that would require an additional circuit breaker as in Question A above.  <br><br>I certainly appreciate any help offered, and thank you for reading this kind of long post!  He's a cheapskate all right, but he's a *GREAT* brother and I want to do good by him.  <br><br>Thank you,<br>Dave<br>

 

[iwire]

Not sure what happened there LowEnergyParticle but I think you may have to try again.

 

[iwire]

Dave I did my best to clean it up.

Hello all, I'm a new member here, and have read through a number of posts in the forum. There are a lot of interesting topics and some very skilled and experienced people here! I'm looking forward to learning a lot.

I've got a little government-work after-hours project going on that I need some advice and clarification on. My brother lives in the Frozen North, and has a small machine shop at his home in an enclosed but unheated room. He's interested in keeping the machines from rusting, but is a notable cheapskate (particularly when it comes to paying the electric bill).

I've designed a small control panel that runs a 120 VAC oil-filled 1.5 KW (12.5 amp) heating element based on a calculation of ambient room temperature, machine steel temperature, and relative humidity. The business-end of the of panel is just a single 25 amp SSR. Since SSRs tend to fail shorted, I've programmed a high-temperature switch to open a contactor just upstream of the SSR: if the room achieves a temperature 15 degrees F above the set point, the contactor will open and drop the heater The contactor will not re-energize until the high temperature condition clears and a reset push button is pressed.

Here's an ASCII version of the power one-line: 20 amp single-pole breaker in the panelboard 20 amp receptacle adjacent to panelboard. receptacle is 8 feet from control panel, and in line-of-sight. ;20 amp plug wired to 2 feet of 12 AWG SOJ cable, spliced in utility box to 3, 12 foot lengths of 12 AWG THHN in conduit between panelboard and control panel. In the control panel, both the hot and neutral conductors land on the contactor (opens on high temperature) 12 AWG neutral conductor wired from contactor to simplex receptacle mounted on bottom of control panel&amp 12 AWG hot conductor wired from contactor to output-side #1 of SSR 12 AWG hot conductor wired from output-side #2 of SSR to simplex receptacle mounted on bottom of control panel 120 VAC oil-filled 1.5 KW (12.5 amp) heating element comes provided with a 2-conductor plug: plugged into simplex receptacle.

Here are my questions:

Is the 20 amp breaker in the panelboard (item 1 above) sufficient for branch circuit protection, or must the control box contain another breaker?

The control box never branches again, there's only the 1 circuit, and the 20 amp breaker at the panelboard is sized correctly. The SSR manufacturer wants a fast I^2T fuse on the output of the SSR. The fuse and block cost quite a bit more than the little 25 amp SSR. The heater will not have the 6x startup surge of a motor, and it also doesn't have to respond to load surges like a motor, so I'm kind of thinking that I should keep a replacement SSR around and not install the I^2T fuse at all.

Is this a violation? The transceivers and logic elements in the control loop are all mounted in the control panel and powered at 120 VAC. I want to provide a 1 amp AG slow blow fuse to power the equipment, and have the fuse powered by jumping from the hot-side of the contactor in Step #4 above.

This seems fine from an ampacity view point, but I'm not sure if I've created a branching that would require an additional circuit breaker as in Question A above.

I certainly appreciate any help offered, and thank you for reading this kind of long post!

He's a cheapskate all right, but he's a *GREAT* brother and I want to do good by him.Thank you, Dave

 

[hurk27]

Nice job Bob:thumbsup:

As to the OP:
That sure sounds like a lot of work when a simple 20 amp line voltage thermostat wired to a receptacle would provide the same control, with maybe a high temp limit mounted close to the heating element would provide for over temp, or just use an electric space heater that has all these features already, 1.5kw is a very common rating for most store bought portable heaters and they have all the safety controls in them including a switch that shuts them down if turned over.

 

[Jraef]

Nice job Bob:thumbsup:

As to the OP:
That sure sounds like a lot of work when a simple 20 amp line voltage thermostat wired to a receptacle would provide the same control, with maybe a high temp limit mounted close to the heating element would provide for over temp, or just use an electric space heater that has all these features already, 1.5kw is a very common rating for most store bought portable heaters and they have all the safety controls in them including a switch that shuts them down if turned over.

Hurk27's valid concern notwithstanding, if you insist on over complicationg this for some reason we are unaware of, you are fine the way it is. No problem tapping off the line side of the contactor to power the controls, it is not a "new branch".

The fuse for the SSR is there ostensibly to protect the SSR from a short circuit on the output. In heater applications, a failure of the heater element or connection to ground will take out the SSR as well. But I think you have found the achilles heel in that concept, the fuse is more expensive than the SSR in most cases like this. I wouldn't bother, 9 times out of 10 the fuse doesn't really clear fast enough anyway. I know it's contrary to what the fuse and SSR people tell you, but that's my experience. I have done a LOT of SCR based power controls over the years and I can count on one hand the times a fuse has protected an SCR. I still recommend them when using SCRs that are clamped in heat sinks because of the amount of down time to change them is less for a fuse, but in the case of a small cheap SSR, the time is likely the same so it's not worth it IMHO.

Also, I would change your logic a bit. Let the SSR do the work, use the contactor ONLY when you power down the entire system to isolate the SSR off line and prevent outside transients from damaging it. You will wear out your contactor doing it this way, that's WHY you use an SSR.

 

[LowEnergyParticle]

Iwire, my original post got tangled up in a cable stretcher. I don't know what happened, but thank you very much for going to all the trouble of sorting it out! I appreciate it very much.

Hurk27, you're probably right; it is kind of silly to have such a complicated system when everyone from Walmart to Sears sells a perfectly useable heater for less than $100.

Jraef, thanks very much for your answers! I will do as you say: A) only BCP in system will be the initial breaker in the panelboard, B) tap the contactor's line-side with a small fuse to power the 4 devices in the control system, and C) don't bother with I^2T fuses on the SSR. We are in agreement on the use of the contactor as a trip device and the SSR as the control element. As you note, that's the point of the SSR. My original post just wasn't very clear: the contactor is normally energized unless a high temperature is seen which would indicate a shorted SSR. The contactor opens on high temp, and manual intervention (reset button) is required to re-energize it.

Thank you all again for your efforts on my behalf!
Dave

 

[weressl]

Iwire, my original post got tangled up in a cable stretcher. I don't know what happened, but thank you very much for going to all the trouble of sorting it out! I appreciate it very much.

Hurk27, you're probably right; it is kind of silly to have such a complicated system when everyone from Walmart to Sears sells a perfectly useable heater for less than $100.

Jraef, thanks very much for your answers! I will do as you say: A) only BCP in system will be the initial breaker in the panelboard, B) tap the contactor's line-side with a small fuse to power the 4 devices in the control system, and C) don't bother with I^2T fuses on the SSR. We are in agreement on the use of the contactor as a trip device and the SSR as the control element. As you note, that's the point of the SSR. My original post just wasn't very clear: the contactor is normally energized unless a high temperature is seen which would indicate a shorted SSR. The contactor opens on high temp, and manual intervention (reset button) is required to re-energize it.

Thank you all again for your efforts on my behalf!
Dave

Just a side comment: all fuses ARE I²t devices, so it is misleading to call for a device by such a name when one means fast acting fuse.

 

[jim dungar]

Just a side comment: all fuses ARE I²t devices, so it is misleading to call for a device by such a name when one means fast acting fuse.

Except that I?t is a common description of fuses which are sized and applied more for their short circuit protection rather than their full load rating (i.e. semiconductor protection).

 

[Jraef]

Just a side comment: all fuses ARE I²t devices, so it is misleading to call for a device by such a name when one means fast acting fuse.

"I²t fuse" in this context is usually used to describe "high speed semiconductor" fuses that still have the inherent I²t time delay but are not technically listed as branch circuit protection devices, meaning you need something else ahead of them. This is often used to mean OTHER THAN Class T fuses, which are Branch Circuit Protection fuses that are higher speed than other BCP rated types but are short circuit only, they are not I²t. So if you try to use them on anything with any inrush current, they need to be over sized significantly and may also need a thermal device in the circuit, i.e. used ahead of a VFD or Soft Starter.

The common issue behind the wording of the SSR instruction manuals is that they want you to use high speed fuses to try to protect the semiconductors, but people were often using Class T fuses with SSRs because they see them used ahead of other semiconductor based equipment such as VFDs and RVSS starters. But that isn't a good application because an SSR does not inherently include any long time overload protection for the branch circuit. However because the I²t semiconductor types of fuses are only listed as "supplemental protection", you need to have a BCP ahead of them anyway. So it comes back to the earlier issue, the fuses are often now more expensive than the SSR, so as long as it's easy to change them out, I don't bother with the fuses any longer.

 

[weressl]

Except that I?t is a common description of fuses which are sized and applied more for their short circuit protection rather than their full load rating (i.e. semiconductor protection).

True, but I still maintain that the common usage has a logical fault. The I²t portion of adjustable protective devices are for inrush and long acceleration of high inertia loads in motor protection adjustments and not short circuit protection. Do you see the inconsistency?

 

[jim dungar]

True, but I still maintain that the common usage has a logical fault. The I²t portion of adjustable protective devices are for inrush and long acceleration of high inertia loads in motor protection adjustments and not short circuit protection. Do you see the inconsistency?

No.

For protective devices, I?t simply refers to the area on the curve where the devices goes from a time delay to operation without intentional delay.
In some cases I want a low trip point (i.e. minimize damage from a short circuit) and other times I want a high trip point (i.e. coordination with accelerating motors).

There are many device which are employed primarily for short circuit reaction time, such as cable protectors, so why the fuse industry has co-oped the term I?t for 'semiconductor' protection is beyond me, but that is what has happened

 

[weressl]

No.

For protective devices, I?t simply refers to the area on the curve where the devices goes from a time delay to operation without intentional delay.
In some cases I want a low trip point (i.e. minimize damage from a short circuit) and other times I want a high trip point (i.e. coordination with accelerating motors).

There are many device which are employed primarily for short circuit reaction time, such as cable protectors, so why the fuse industry has co-oped the term I?t for 'semiconductor' protection is beyond me, but that is what has happened

Yes.

http://www.geindustrial.com/publibrary/checkout/DES-098A?TNR=Time Current Curves|DES-098A|generic
http://www.geindustrial.com/publibrary/checkout/GES-9913?TNR=Time Current Curves|GES-9913|generic