# NEC 670.6 - Surge Protection for Safety Interlocks



## splatz (May 23, 2015)

From a ppt that @paulengr recently posted - 

New in 2017 NEC 

670.6 Surge Protection. Industrial machinery with safety interlock circuits shall have surge protection installed. 

Is that something that has to be incorporated in the design of the machine itself, or something you have to include when installing the machine?


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## Peewee0413 (Oct 18, 2012)

I thought design.


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## Almost Retired (Sep 14, 2021)

if it is listed, then that should be in the design
who is to say that it Will be installed when the machine is installed?


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## paulengr (Oct 8, 2017)

With where it is located it’s not a Listing requirement and doesn’t say where the SPD has to be located. What’s worse the justification is based on an opinion survey, not factual evidence.

Still it’s a joke. Drop a $1000 SPD in and move on. If you already have one somewhere in the plant that meets the requirement such as it is.

Realistically it’s another reason E-Stops are a really bad idea. Three issues with E-Stops:
1. Realistically what does it do? In terms of safety if you do a safety evaluation you need to have some way to address hazards. As humans have very poor error rates (about 10% in nonstressful conditions, 40% in emergencies) an E-Stop can never meet any safety requirements where performance matters. I mean if the difference between someone getting injured and not depends on a system with roughly a 50% failure rate, is that realistically giving anyone comfort? Anyone doing accident investigations knows this comes up time snd again. The issue isn’t the E-Stops themselves, it’s the human operator.
2. OSHA does NOT require them except on hydraulic power presses. But if you do have them lots of additional requirements follow such as the above NEC. If you just call it just Stop or Process Stop though you get your button with no requirements.
3. Ordinary start/stop circuits are no less reliable than E-Stops (again the human error rate drives this), so E-Stop circuits as safety circuits are stupid,
4. An E-Stop demonstrates the safety evaluation required by OSHA is defective. Most safety evaluations use a E-Stop as a “catch all”. If there are other hazards you need to address them, period. If the regular machine controls won’t cut it, neither will a second button. Caveat here: if you have a system that is entirely networked, it is occasionally useful to have a hard wired stop button. I mean if everything is on HMIs that are networked and you lose the network how do you safely shut everything down? That’s where A hardware stop button is useful.
5. To be effective an E-Stop shouldn’t just be coasting to stop. We need bent ripping, brakes squeaking, CRASH type stops. An E-Stop should be capable of equipment destruction. Hence it may not be something anyone wants.

The accidents I’ve seen with only E-Stop protection are crazy. Operators standing there flipping out over what they see or fleeing the scene are common. Or arguing with each other on hitting the button. Or just hitting EVERY button except the one that works. Or it doesn’t work or work as expected. I’ve never seen E-Stops actually work in an emergency.

Plus even if you have a real safety system the vast majority work on 24 VDC and there’s a very good reason why. AC switching generally takes up to 16 milliseconds, partly waiting for an AC zero crossing. DC responds typically in 3-4 milliseconds at most. Plus it is power controlled and arc free so reliability is much higher. DC systems are fed by DC power supplies that naturally protect them from damage. So I see this entire justification as a total joke.

So it’s pretty easy to neuter 670 for most machines,

Again why isn’t NFPA sued over this one?


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## derslo (11 mo ago)

In spite of the fact that wellbeing gadgets are tracked down fundamentally in modern settings, NFPA 70 670.6 applies to different applications too, for example, server farms, emergency clinics, and inns, where security hardware is set up to respite or stop apparatus. Any place a security gadget is utilized, this prerequisite applies.


snaptube vidmate


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## paulengr (Oct 8, 2017)

derslo said:


> In spite of the fact that wellbeing gadgets are tracked down fundamentally in modern settings, NFPA 70 670.6 applies to different applications too, for example, server farms, emergency clinics, and inns, where security hardware is set up to respite or stop apparatus. Any place a security gadget is utilized, this prerequisite applies.


Show me a server farm not running on UPSs and without a good surge arrester to protect the SCRs from self commutation. Same with hospitals. It’s regular industrial controls..

Show me an inn where the panic hardware doesn’t allow opening a door. And for that matter this happens all the time. They have easy ways of bypassing the electronics with mechanical keys. Door controls are 24 V.

In all cases due to number of doors and costs the lock system is networked and 24 V at most. Networking is two wire serial, BAC net, LON, Modbus, etc. SPD if it exists would be GDT.

All 3 are customers. None have any reason that an SPD does anything.


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## SWDweller (Dec 9, 2020)

When I was building shovel boxes, up to 7200vac. The owner wanted a remote on off switch on a cord. I figures out how to use the gfci recpt already there and put the remote switch on the ground fault. Figuring miners were in all sorts of weather doing operations.

FYI I did not ask I just did it under basic safety rules. I actually got an atta boy and they sent us more boxes for the upgrade.


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