NEC Requirement for Motor Control Circuit

The NEC says no such thing. Don't know wherre you got that from, but it was incorrect. If you want help, please describe your entire situation.

Good information. Makes sense. i think US customers and maybe NEC prefer devices like this one, with the blades visibly open when OFF ?

.

?

US customers and maybe NEC prefer devices...

No. Because of OSHA requirements,such cabinets can never be open, and therefore only the handle can indicate the on/off status.

No it means that the disconnect for the controller applies to the one necessary for the motor if it is with in site of both. If both the controller and the motor are not with in site of each other. Then a disconnect means have to be provided for both.

Also the disconnect does not have to be in site of the motor or equipment. As long as the disconnect can be locked in the open position.

It's a good idea in practice though to set it with in site.

"I understand that NEC required a motor control station within sight of the motor. The question I have is, does the control station have to break the control circuit directly? Does the NEC allow the switch to send the signal to the DCS and the DCS will stops the motor? "

Control station and disconnect are NOT the same thing. He is referring to a control circuit issue, to which there is no NEC requirement. He may have been mixing up the issues in his mind, but that reference to the LOTO disconnect, however useful, does not actually answer the question he asked.

(msamad and kkvsridhar; no offense meant to your responses, I'm being pedantic because I get tired of people not knowing how to ask a question and then getting the wrong answer)

Oh, no offence taken of course

Such debates actually enrich a layman's (me ) knowledge even if the original post is inane (i am not saying this one is, OP). It is an academic thing for me since i live in an IEC country. i felt that US standards are oriented towards the user (since UL wants the safest stuff for people) while IEC's decision makers come from Industry. i have seen some small vested interest situations in the BIS meetings which i attended. The customer definitely took a second place.

So it was refreshing to see things like a safety switch with visible isolation in sight of the motor one time when i was in the USA. When one is next to piece of machinery which is going nuts, one looks frantically for an emergency switch whose handle one can pull to shut off the darned thing.... NEC or not

fyi KVS - your live in a country where NEC2011 has been released by BIS . . .

So, is this US code based? i am afraid i don't own any Standards nor Codes.

NEC 2011 is for India just like its predecessor NEC 1985 was . . . . all installations will align with NEC 2011 as it gets adopted and harmonized with local supply rules and building rules . . . difficulty is the high variance between building rules in different municipalities and states . . . we are trying through our trade organisations and professional institutions to bring down variance . . . hope to use support from all industry, trade and consumer quarters . . .

Ah, i see.

Anyway, the NEC of USA specified (as my American friends told me in 1982) that (a) Safety switches are to be used as emergency switch-off devices, and should have visible open gap between moving and fixed contacts, and (b) every moving machinery should have an emergency switch within sight.

i thought this was an excellent rule. i can't say whether what my American friends told me was true or not.

There is a difference between a emergency shut off switch and a disconnect.

The device you describe is a disconnect it is there for service to the device. The emergency shut off switch which is usually on the control panel just stops the device. Not that the disconnect can't be used in an emergency. But the emergency stop switch should not be used for service.

In IEC, the device is called a "Switch-disconnector". The "switch" means that the device can open a circuit at an overcurrent level higher than LRA. The "disconnector" means that it is also capable of providing safe isolation. In other words, such a device can be used as a "disconnect" when attending to maintenance on the downstream equipment. (Such maintenance is done when the motor starter is in the Off condition anyway), and is capable of switching off a substantial overcurrent in an emergency.

Such European devices are being used in USA nowadays, through ABB and Socomec who are branding their products for Cooper Bussmann and Ferraz Shawmut.

Spent a few years installing those devices. Here they are referred to as a service disconnect. They can be with or without overload protection. Have been used for at least the last 40 years I know of nothing new. Maybe longer the gas station in worked in as a teenager had them on the wall for all the pumps. The station had been there before I was born.

Do not confuse an emergency switch with a disconnect switch. An emergency stop switch causes the control circuit to drop out, stopping all machinery in the process/line. A disconnect switch prevents power from getting to a specific motor regardless of what the control system wants it to do.

When one is next to piece of machinery which is going nuts, one looks frantically for an emergency switch whose handle one can pull to shut off the darned thing.... NEC or not

An "E-stop" (emergency stop control rope or button) is what one should be looking for* to quickly stop machine motion. These are wired into the control circuits and not related to the disconnect switches. Often, the disconnect switch cannot be reached quickly enough if someone is being sucked into a machine. The disconnect switch is there for lockout-tagout purposes, not to stop a motor in an emergency.

Disconnect switches are typically not equipped with arc quenching provisions, so should not be operated with the motor under load.

* (and actually should already have looked -- if you are working near a machine, you should know the location of the e-stops, and as an operator should have tested them at the start of your shift.)

In the UK, any requirement for an isolator at or near the motor will break the control circuit first, then break the phases to the motor pending the fitting of a padlock. If fitted, an emergency-stop pushbutton will be wired in series in the control circuit.

There is no requirement on pushbutton stations' locations. Any number of installations simply don't have them.

As ozzb said, there are acceptable exceptions to the rule, but it is still considered a "best practice" to put a disconnect withion sight. Also, the rules originally came from OSHA, the Occupational Safety Hazards Association that was involved in "enforcement" of safety standards, but was rendered toothless under the Bush Administration. So in 2005, the NEC adopted the same rules to make it enforeceable at the local electrical inspector's level. Technically though, if an end user fails to have a LOTO (Lock-Out-Tag-Out) disconnect device within sight of the motor, and does not prove that one of the exceptions applies, then OSHA can fine them even if an electrical inspector allowed it. Thirdly, there is another code similar to the NEC (which is technically the NFPA 70), called the NFPA 79 which specifically covers machinery safety issues and can be enforced by the local fire marshals, who may or may not accept the exceptions allowed by the NEC or OSHA. So with the potential of getting dinged by 3 separate agencies, most people are loathe to take the chance just to avoid the slight increase in installed cost to put in a LOTO device. Hence the "best practice".

I'd like it better if it said with in site and throwing distance of the tools at hand.

Generally an emergency stop button(big red top) to stop the machine should be near the machine operator. Also an isolating switch should be near the motor(in sight) after the starter, to isolate motor/machine for maintenance so that nobody can switch it on. Then a question arises- when the starter is under maintenance and if the electrician normally switch off the circuit from the switchboard if switchboard is not in sight from the starter can someone switch the breaker on?. Do we need another isolator before the starter?. Is a notice "danger,do not switch on" on the breaker sufficient if it is not lockable?. What does the regulations say?.

Can a lockable mushroom head emergency off push button located next to the motor be used in place of switch disconnector/ disconnector/LOTO placed in visible location to motor?

A big, yellow/red emergency pushbutton can still interrupt only the control circuit. Never the power circuit. If the contactor contacts have welded shut, nothing happens. Motor keeps on getting supply. A manual, visible blade safety switch is the best.

A manual disconnect switch with open blades and no arc quenching is dangerous to operate with the motor under load, it can blow the enclosure open and start a fire or electrocute the person pulling the handle.

No!

MSAMAD hit this one very nice! 3 years ago we were given a task of adding an AC drive to a conveyor system in a cement plant on their feeder system to the barges.

We arrived, MCC bucket was locked out/tagged out. Machine was also locked out tagged out. (Or we thought.) We did the drive install, which also meant changing several items on the machine motors, adding encoders, changing limit switches, adding various sensors and coupling therm all to the drive and the DCS system.

Start up, machine did nothing... Spent days testing. Days later guys complaining a system in the middle of the line has been acting up and dumping product on the belt blocking it.

We finally realized the Lockout/tag-out was the wrong MCC bucket, the machine we were working on was Live 480 3Ph and could have been active at any time via the DCS system. Could have been much worse since most of their equipment was all 4160 3Ph. Luckily nothing happened electrically bad.

Also, the machine was on the 2nd floor, MCC was on first floor and maybe 300 feet away. No disconnects, no E-Stops to be seen. Actual machine controlled by New VFD, 1900 feet away from MCC Bucket.

Value of lesson: ALWAYS CHECK LO/TO! Never ever take anyone's word for granted especially if it comes from an engineer!

I still say, all engineers must work at least 3 years in the field before they sit behind that desk and dictate what takes place in the field with those CAD DWG's and what ever else they say! Got to have that hands on!!

It is worth noting that even if you had "checked" the LOTO status, you would still not be in compliance. If you are working on a machine your locks must be on the disconnects.

If working under group LOTO rules, then the keys for the various disconnect locks must be locked in a cabinet that is locked with all the individual locks of the workers, so that access to the keys cannot be obtained without each worker removing his lock.

Part of the LOTO process is to test* that the machine (or part of it you are working on) is in fact dead. In many companies, failure to do so gets you fired... if not killed.

* Unfortunately, in many companies and in many contracting outfits, the workers are clueless re how to test. Too many assume that because the motor does not start when the start button is pressed, that they have turned off the correct disconnect. Perhaps they have only turned off the control circuit.

No, just a disconnect that opens the power circuit so it can be safely locked out.

In addition an emergency pushbutton with mushroom head near the machine operator is essential.

The purpose of the emergency stop pushbutton is only to save the machinery from self destruct. Workers should be protected by physical and procedural bariers.

In most jurisdictions, this is not true. Many machines are equipped with emergency stop buttons (and ropes, belly bars, etc) that perform a safety function. The regulations come from many sources and for many reasons, but there are many machines that cannot be so thoroughly guarded as to eliminate the possibility of operator injury (or injury of bystanders). Many lives have been saved (and injuries avoided or lessened) by rapid use of e-stop buttons.

You are getting so many different answers from many people. Some are close. Some are right, and some are just dead wrong. I think you should consult a locally licensed electrical Professional Engineer to get the straight story. He can inspect your situation and advise you best.

The NEC requires a Disconnect switch within sight of the machine. This is so that someone working on the machine will not be injured by an unanticipated start-up. If not within sight of the apparatus the disconnect must also accept a padlock to prevent some well-meaning idiot from starting it and removing your (insert appropriate body part here).

To sum up, a good practice will be to have a lockable disconnect switch with visible blades in sight of the machine. In addition an auxiliary switch from this disconnect feeding back to MCC thereby interlocking the starter. Further a mushroom head emergency stop button near the machine for fast action if the machine has to be switched off in a hurry.

The OP is long absent and the rest of us are like the fabled group of blind men arguing over the description of an elephant by examining only the one part we can touch...

This subject has way too many variations to be determined from afar in a general way, there is no grand unifying rule.