NEC compliance with regard to TVSS fusing 1

[seamus000]

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.

 

[Sparkyonline]

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.

 

[peebee]

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.

 

[Sparkyonline]

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

Good info.

 

[seamus000]

All, thank you for your comments.