Can a 5500 watt resistive heating element be manually controlled by a general duty disconnect switch. The disconnect switch would be used to turn the heating element on and off under load. The unit does not have a t/stat or an on/off switch. Would the disconnect switch hold up after numerous throws?
disconnecting 5500 watt resistive heating element from power source
- Thread starter jmbrwn
- Start date
- Status
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I suspect even a heavy duty safety switch will wear out with numerous throws over time. Why not use a button with a contactor. Not sure if a disconnect switch is rated for switching
Is there a reason that the voltage was omitted? One would "assume" that it is 120v which a boat load of amperes, almost 46a. As such would it be 240v which wouldbe about 23a? There would be no issue should the switch selected be rated for the voltage and current since this is purely a resistive load.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
The UL98 test standards for disconnects are stringent with regard to the number of operations under load. In reality, UL98 does not differentiate between GD and HD, the test standards are the same. I believe it is 6000 total operations at rated load, something drastic like that, and then up to 4000 operations unloaded, but being operated once every 20 seconds or so (it's been a long time).
The difference between GD and HD is about two things: maximum withstand capacity and long term reliability. The GD switches are intended for residential / light commercial use so are typically limited to 240VAC, and with the right fuses, can be used in 10kA-100kA AFC applications. HD switches can go up to 600VAC and up to 200kA AFC with the right fuses. Basically, GD switches are made cheaper for those who would hardly ever use them and never abuse them.
Both meet the MINIMUM standards, the HD versions exceed those standards.
You didn't say the voltage, so ASSuming 240VAC single phase (because you said GD switch), a 30A fused switch would be fine for this. I'm not saying it's a good idea to not have more control over it than a simple safety switch, but at a bare minimum, that would be acceptable.
The difference between GD and HD is about two things: maximum withstand capacity and long term reliability. The GD switches are intended for residential / light commercial use so are typically limited to 240VAC, and with the right fuses, can be used in 10kA-100kA AFC applications. HD switches can go up to 600VAC and up to 200kA AFC with the right fuses. Basically, GD switches are made cheaper for those who would hardly ever use them and never abuse them.
Both meet the MINIMUM standards, the HD versions exceed those standards.
You didn't say the voltage, so ASSuming 240VAC single phase (because you said GD switch), a 30A fused switch would be fine for this. I'm not saying it's a good idea to not have more control over it than a simple safety switch, but at a bare minimum, that would be acceptable.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
I found some old notes:
The unloaded test is 4,000 operations at a rate of 6 per minute, so once every 10 seconds (not every 20).
The 6,000 ops under rated load has no time or duty cycle assigned, just the total number.
The unloaded test is 4,000 operations at a rate of 6 per minute, so once every 10 seconds (not every 20).
The 6,000 ops under rated load has no time or duty cycle assigned, just the total number.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
That is not all that high of a number, seem to recall NEMA contactors have a rating of at least 1 million operations.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Correct, hence the manual safety switch not being called a manual motor starter, despite the fact that it can be used that way.
I have an ongoing issue at a food plant right now where they do in fact use disconnect switches in MCCs on motor loads at least 6 times per day during CIP procedures, and they wear out in about 3 years. That's why I had to investigate this. Their safety people insist that motors be locked out during automated CIP, even though people are not directly interacting with the machinery. Then the electricians get crap from management for having to replace dozens of disconnects every year (they have over 500 installed in the facility), which involves added down time when they fail. The managers ask around in their industry about how often other people have to replace disconnect switches in MCCs and they hear that it's rare, but they fail to understand that what they are doing is beyond the norm.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
At least they are not trying to break the load with them, yes?
At that rate the 6000 UL spec limit is reached in about three years, so it looks like they are not built any stronger than they are required to be.
What is the failure mode you see? High contact resistance and burnout? Or mechanical failure?
At that rate the 6000 UL spec limit is reached in about three years, so it looks like they are not built any stronger than they are required to be.
What is the failure mode you see? High contact resistance and burnout? Or mechanical failure?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
My experience from food processing is mostly with the dairy industry. Seems most motors I came across needed ON/OFF/CIP controls as they would run them during CIP but the CIP controller would turn them on at the right time during the cycle. That was especially typical for pumps, other equipment didn't always need to run when cleaning though. Not sure why they think they need to lock off equipment when in CIP, nobody is supposed to be inside equipment during such operation though depending on what hazards may be introduced disconnection for hand cleaning makes sense.
- Status