A discussion is going on the site that why it is required to install disconnect switches near the motor control center?? The answer is simple for maintenance reasons, but why it cannot (if cost allows)be a circuitbreaker instead of disconnect switch that will facilitate double line of defense even when the main breaker fails to trip due to any reason?? There are no substantial explaination except to say that its not permissable as per code??
Why disconnect switch to be preffered overCircuit breake
- Thread starter pasha
- Start date
- Status
- Location
- Illinois
- Occupation
- retired electrician
Re: Why disconnect switch to be preffered overCircuit breake
I don't know of a code rule that says you can't use a circuit breaker as a manual means of disconnect. As long as you can operate the breaker without opening the enclosure, I see no reason not to use a CB.
Don
I don't know of a code rule that says you can't use a circuit breaker as a manual means of disconnect. As long as you can operate the breaker without opening the enclosure, I see no reason not to use a CB.
Don
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Why disconnect switch to be preffered overCircuit breake
Circuit Breakers are specifically recognized as a disconnecting means in 430.109(A)(2)
Circuit Breakers are specifically recognized as a disconnecting means in 430.109(A)(2)
luke warmwater
Senior Member
Re: Why disconnect switch to be preffered overCircuit breake
I actually can get a CH 200a breaker in a 3R enclosure for less than I could get a 200a 3R fusible disconnect.
I actually can get a CH 200a breaker in a 3R enclosure for less than I could get a 200a 3R fusible disconnect.
caosesvida
Senior Member
Re: Why disconnect switch to be preffered overCircuit breake
maybe the confusion is over the fact that you have a little more leeway with fuses than circuit breakers. ie. one time, and time delay?
maybe the confusion is over the fact that you have a little more leeway with fuses than circuit breakers. ie. one time, and time delay?
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Why disconnect switch to be preffered overCircuit breake
A motor disconnecting means is not required to have any overcurrent protection functions. It's purpose is isolation.
In general, for 100hp and below it is required to be able to interrupt running, but not necessarily locked rotor currents.
A motor disconnecting means is not required to have any overcurrent protection functions. It's purpose is isolation.
In general, for 100hp and below it is required to be able to interrupt running, but not necessarily locked rotor currents.
jcormack
Member
- Location
- Pennsylvania
Re: Why disconnect switch to be preffered overCircuit breake
The main reason for safety disconnects in sight of a motor-driven unit is for emergency shutdown or as a lock-out device for service work. Some industries are more severe in their requirements than others. The reason the disconnect should be mounted within sight of the unit & labeled properly is so that the correct device has power removed. My industry is governed by MSHA - and as such "visible" disconnects are required when doing electrical service - a circuit breaker by design does not provide "visible" indication that the power connections are severed - thus an electrician must verify with a volt-meter that the motor leads are de-energized. I only use disconnects with a viewing window, thus a non "electrical" person can do mechanical maintenance without the need for an electrician to be present. Stop switches & pull-cords are not acceptable lock-out devices for doing service work as wiring shorts or faulty switches (or PLC forces)can allow machines to re-start, hence the need for a local lock-out device that prevents motor starting. In most of our surface facilities the MCC room with the MCP devices is not close to the actual device, sometimes 4-5 floors and hundreds of feet away.
The main reason for safety disconnects in sight of a motor-driven unit is for emergency shutdown or as a lock-out device for service work. Some industries are more severe in their requirements than others. The reason the disconnect should be mounted within sight of the unit & labeled properly is so that the correct device has power removed. My industry is governed by MSHA - and as such "visible" disconnects are required when doing electrical service - a circuit breaker by design does not provide "visible" indication that the power connections are severed - thus an electrician must verify with a volt-meter that the motor leads are de-energized. I only use disconnects with a viewing window, thus a non "electrical" person can do mechanical maintenance without the need for an electrician to be present. Stop switches & pull-cords are not acceptable lock-out devices for doing service work as wiring shorts or faulty switches (or PLC forces)can allow machines to re-start, hence the need for a local lock-out device that prevents motor starting. In most of our surface facilities the MCC room with the MCP devices is not close to the actual device, sometimes 4-5 floors and hundreds of feet away.
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Why disconnect switch to be preffered overCircuit breake
Disconnects are NOT for emergency shutdowns. Emergency shutdowns are a control issue. Disconnects are a maintenance issue.
I've been involved with this as a former Principal of both CMP 11(1996) and the 70E TC (2000). I personally pointed out several of the FPN No.1 "examples" to my former colleagues on CMP 11.
I also pointed out there are many cases in the petrochem industry (my primary background) where you definitely want the manual emergency shutdown actuating device "out of sight."
Disconnects are NOT for emergency shutdowns. Emergency shutdowns are a control issue. Disconnects are a maintenance issue.
I've been involved with this as a former Principal of both CMP 11(1996) and the 70E TC (2000). I personally pointed out several of the FPN No.1 "examples" to my former colleagues on CMP 11.
I also pointed out there are many cases in the petrochem industry (my primary background) where you definitely want the manual emergency shutdown actuating device "out of sight."
stew
Senior Member
- Location
- federal way,washington
Re: Why disconnect switch to be preffered overCircuit breake
jcormack The code does in fact require circuit breakers to indicate on/off see 404.7 where this is specifically required. Safety proceedure that I teach all newbies and apprentices require that voltage be checked on any device that is seemingly deactivated wheter or not it indicating off . Never know when someone mite bottom feed a switch or breaker. A voltage indicating device of some sort is the electricians first line of defense against shock.
jcormack The code does in fact require circuit breakers to indicate on/off see 404.7 where this is specifically required. Safety proceedure that I teach all newbies and apprentices require that voltage be checked on any device that is seemingly deactivated wheter or not it indicating off . Never know when someone mite bottom feed a switch or breaker. A voltage indicating device of some sort is the electricians first line of defense against shock.
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