E-Stop Vs Stop & VFD Power Removal

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FattyArbuckle

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I have two questions regarding E-Stop usage.
For 10+ years I have incorporated two ?rules? regarding E-Stops.

  1. E-Stops cannot be used as a standard stop button. The thought is that this defeats the purpose with regards to training employees. If it is an emergency we want employees to act appropriately and have trust that the equipment will act appropriately with an immediate response by stopping and controlling energy sources.

    Does any code prohibit an E-Stop being used as a regular cycle-stop button?
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  2. In Europe (and maybe other areas) they have safety-rated Variable Frequency Drives (VFD?s). My understanding is that USA code requires the removal of power from non-safety rated VFD?s in an E-Stop condition ? the VFD software cannot be used to assure equipment stoppage. (I don?t think any USA code addresses safety-rated VFD?s yet.) This of course, requires a relay before the VFD.

    Are we required by code to remove power from a VFD under E-Stop (as we cannot rely upon software in an emergency condition) ?
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I am getting pushback from one of our remote USA plants on the installation of equipment (designed in-house) and the need to follow the above two rules for E-Stops.

Can anyone provide code information (NFPA, IEEE, NEMA, OSHA, etc.), and paraphrase the code if you have time, to support my approach?

Thanks!
 
1. I am not aware of any code that prohibits such use however from experience this is not an electrical question but a safety department question. Our safety department prohibits the use of any emergency device or feature for any use other than an emergency.

2. E-stops have no standing in the US per se. Their use is driven by common industrial practice. The best source for such is a voluntary standard your company should adopt: NPFA79 Electrical Standard for Industrial Machinery 2007 Edition.

So paraphrasing from NFPA79:2007

9.2.5.3 Each machine shall have a Category 0 stop (Immediate Power Loss) which is typically the main machine disconnect.

9.2.5.3.2 Other stops shall be provided by risk assessment.

Note: From the NEC Article 110, you cannot use a device outside its listed purpose therefore non-safety devices of any type cannot be used as safety devices.

9.4.3 Control systems may incorporate software and firmware based safety devices provided they meet the requirements of this section. Tamper prevention, memory retention and verification, failure mode, and listed for the purpose are some of the requirements. So yes, under this provision safety-rated VFDs can potentially be used.

The NFPA website has a way to preview codes if you register with them: http://www.nfpa.org/aboutthecodes/list_of_codes_and_standards.asp?cookie_test=1
 
FattyArbuckle said:
Thanks - this will give me some ammo!
(Why do we have to fight for a basic safety function?)
Click to expand...

Think about my statement to 2 above. Estops have no official standing. Peeps tell me every machine must have an Estop. That's simply not true but it is a prevalent myth.

The correct thing to remember about ALL safety is this:

The correct application of safety can only be determined by a risk analysis performed by someone familiar with the work.
Click to expand...
 
Yes - Risk Assessment is important.
In this case it is large ~10Ft tall rotating equipment.
People frequently interact - opening doors & filling, etc.
(This is not maintenance - that would entail LOTO.)
While it is hard to quantify the likelihood of component failure causing unexpected rotation, the cost (severe injury / death) is high.
E-Stop and cutting power to the VFD seems appropriate.
 
Without knowing just what you are doing it is hard to know what is appropriate. I personally do not have an issue at all with using an "e-stop" button to shut down equipment.

You are supposed to test them anyway. What better way to test them then to use them regularly. It is not like they are going to wear out.

I am not a big fan of installing contactors on either the line or load side of VFDs. These days many VFDs can be ordered with a safety circuit that is built into the drive that can be tripped that is close to being equivalent safety wise to a line or load contactor. I would head that way if I thought it was necessary.

Keep in mind that a lot of applications do not have much in the way of danger associated with them that an e-stop is going to mitigate.
 
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