×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

NEC compliance with regard to TVSS fusing

NEC compliance with regard to TVSS fusing

NEC compliance with regard to TVSS fusing

(OP)
NEC compliance with regard to TVSS fusing

With regard to NEC Articles 280 & 285:

1) Is fusing of TVSS circuits in compliance?  If so, what are the acceptable sizing guidlines?  Blown fusing, while protecting the TVSS equipment, would leave the equipment exposed to future spikes, which appears to be in violation of the articles.

2) What are the seperating differences between "surge arrestors" and "TVSS"?

Thank you in advance.

RE: NEC compliance with regard to TVSS fusing

Transient Voltage Surge Suppression, in its basic form, is NOT fused.

For said protection in given system, esp high-rise or medical installation, same is accomplished by separate grounding conductor from bonding point(s) in the system (preferrably at the main, but acceptable at points through the system i.e. sub-panels).  This ground conductor will serve as bonding all portions of interfacing equipment/peripherals (e.g. cable tray for voice/data) and prevent the possible for any differential in ground impedance.

You are correct in your assumption that fusing would pose the scenario for this protection to be defeated.

Fusing &/or disconnecting means would enter into the picture when utilizing electronic equipment for TVSS, such as power conditioners, in-line surge arrestors, and the like.  However, in such a case, all power would be broken to the unit, and no bonding path interrrupted.

RE: NEC compliance with regard to TVSS fusing

I'm not quite sure if Sparky & I are saying the same thing.  So here's my take on it:

1A)  Overcurrent protection (breakers or fuses) is generally not required, but it is permitted for most general installations (hospitals & life safety are out of my area of expertise, I'm not talking about them, I'm talking about typical office or data center installations).  Breakers are often used to connect TVSS units to panelboards, and breakers or fused switches are often used for connection to switchboards/switchgear.  The primary function of these devices is to act as a disconnect for the TVSS, though, rather than as overcurrent -- you'd be bummed out if you had to power down your main switchgear to replace a stupid TVSS, so the disconnect is a really good idea.  

1B)  In my opinion, the sizing of such breakers or fuses is best left to the TVSS manufacturer, and they are usually happy to recommend a size.  The continuous rating of the fuse/breaker has nothing to do with the selection, I know that much -- we are not talking about continuous currents here, we're talking about durations of micro- or nano-seconds.  A standard 15-amp breaker or fuse will permit thousands of amps to flow for that duration without tripping.

1C)  If the fuse blows or breaker trips, disconnecting your TVSS is not such a big deal, because the disconnect was due to a TVSS failure -- the fuse blew because your TVSS turned into a bolted short circuit.  If you didn't have the fuse or breaker in there, you'd trip your main breaker instead.  So not only would you have lost TVSS protection, you would have lost power too.  While such external overcurrent protection is not required, it's not a bad idea either.  Also, keep in mind that most TVSS's are internally fused.  So even if you don't put external overcurrent protection in, you still have the same potential problem due to the internal fusing.

2)  The main conceptual difference between a surge arrestor and a TVSS is the size of the device and the location in the power distribution system.  Surge arrestors are typically big beefy devices meant to protect entire systems from big surges such as lightning strokes.  They are permitted to be installed on the utility-side of your main service disconnect switch.  They handle the big surge problems, and knock them down to more manageable levels.  TVSS's are used for baby spikes, they further reduce whatever lightning impulse makes it by the surge arrestor and they mitigate problems from big motors starting.  TVSS's will clamp the voltage to a level closer to the normal line voltage.  It is often a good idea to use both surge arrestors and TVSS's together.  Surge arrestors may not be justified in some applications -- for example, if the utility line is entirely routed underground, there's little chance that a big spike could ever be imposed on the utility source.

Hope this helps.

RE: NEC compliance with regard to TVSS fusing

Peebee and myself are in agreement.  While I emphasized on a grounding path hardwire system, he emphasized on the electronic component version.

Good info.

RE: NEC compliance with regard to TVSS fusing

(OP)
All, thank you for your comments.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close