We do see type 1 SPDs (MOV) installed as shown in the course. The typical type 1 MOV installation is in parallel with the HV primary of an external transformer or its associated HV cable and without a fuse or breaker. The reason a type 1 installation is acceptable without protection is that when the MOV fails it does so in open air and presents little risk to the supply. An outage however is required to replace these lightning arrestors when they do fail. The MOVs are chosen with a high enough voltage rating so that they do not conduct at the grid surge level of voltage spikes. These MOVs consequently do a great job and rarely fail as they are being used correctly to make the grid more robust. The grid robustness comes from protecting the insulation of the transformers and the cables insulation from punch through failure due to lightning.

These external events are exactly where MOVs are best installed and without the need for a fuse or breaker.

Regards, Paul

After years of poor reliability findings in critical power systems like UPSs (AC) and rectifiers (DC), we were introduced to the Transtector line of Silicon products, which we coordinated with the existing MOV installations. Failure rates reduced with Silicon, so we continued with Transtector.

We do already use MOVs at the point of entry applications to stop short pulse lightning entry. We use only Silicon to protect critical equipment within a building from grid and internal reflected long pulse surges.

In both cases, we use a breaker rated to the supply fault current for isolation and overload/fault protection. The reason is that the thermal isolation in the SPD is not rated for the short circuit current. It must use an upstream fuse or breaker to cascade with so that the energy let through does not blow the SPD off the wall during a failure of the SPD.

An SPD does not show or have a fault rating (clearance) it only shows a conduction rating. The clearance rating has to equal or be more than the available supply fault rating.

Regards, Paul

Thanks,
Ken

Boby’s original posting raises concerns not about the surge current capacity of an SPD, but of it’s SCCR. It looks like the SPDs he has been using have limited to no SCCR. This is a big issue for IEC style SPDs. Most of these style of SPDs rely on spark gap technology to surge protect an electrical system, and in most instances, they have limited to no internal fault current protection. When they short circuit, they definitely will compromise the electrical system.

The dilemma is to provide a SPD that safely disengages from the electrical system while allowing the client to achieve lightning protection certification for or similar to UL 96A. All of our SPDs were designed using input from engineers at Siemens and Schneider, and this resulted in the development of our safety control circuit that incorporates thermal protection with surge rated fuses. This circuit allows allows our SPDs provide excess surge current capacity up to a 1000kA while safely disengaging from the electrical system during a fault condition. I think some of the confusion is based on how an MOV fails.

During end of life, SPDs tend to short circuit. The common assumption is that when short circuiting occurs, the SPD will draw the full available fault current. If this was true, fault current protection would be rather simple. However, SPDs comprised of either MOVs or SADs or both act like voltage sensitive resistors. Depending upon the applied voltage the MOVs/SADs will short circuit soft or hard. In a majority of instances, SPDs will draw a small amount of fault current. If this fault current is in the range of a safety control coordination blind spot, the SPD could fail as Boby described.

How do we know this scenario occurs? Because, Underwriters Laboratories keeps receiving reports of field failures, and they keep updating their fault current testing to root out these blind spots. With UL 1449 3rd edition, SPDs need to pass low, intermediate, and high fault current testing in order to pass 1449. IEC style SPDs can not pass this test and meet the the Type 1 I-n requirements without the use of external safety controls. This them limits their Type rating to “Type 2″ as well as limited I-n ratings.

I agree with you assessment concerning long pulses versus lighting pulses. Long wave pulses coming from the grid tend to be multi-cycle, which is an eternity and outside the scope of a transient SPDs are designed to mitigate. Thus, fault current protection needs to be coordinated to take the SPD off line when exposed to electrical disturbances outside the scope of electrical surges or transients. Since 1998, our SPDs incorporated coordinated full spectrum fault current protection, which makes our SPDs unique in the industry.

I coved this topic in greater detail in Siemens Step 2000 – Basics of Surge protection training module that I authored. Check it out and give me your thoughts.

http://www.usa.siemens.com/step

Thanks,
Ken

If so, what do you do? You always must have a fault/overload clearing device, to isolate the suppressor when it fails, otherwise you will escalate the fault (we have investigated fires to prove this). Suppressors DO fail. When they fail, how do you isolate them to replace them?
If no isolation is installed then the supply must be powered off to be serviced safely. In a critical site, this power down is unacceptable.

When installing a suppressor, whether MOV for short pulse (lightning) or Silicon for long pulse (grid), you need an isolator whether by a fuse or by a breaker. That isolator/overload device needs to be rated to the fault capacity of the input supply. For example if the supply transformer feeding a switchboard can produce 75kA then you must use a 75kA fuse or breaker to protect the SPD from the failure mode event. The rating of the SPD is NOT the same kA. A 150kA MOV is only rated at this short pulse for one time. The actual reason an MOV has a high kA is that it will survive many smaller surges. Short pulse surges do not make it into a building due to the inductance of the cabling impeding high speed pulses (high frequencies). Trying to pass even a few kA (less than 5kA) will destroy the cable insulation first and not the MOV. In effect the cable inductance saves the MOV from the lightning.

The reason why an MOV fails, is that it is exposed to long wave pulses from the grid. Long wave pulses degrade a 150kA MOV into approximately a 3kA single event suppressor! This data is real and published by the MOV manufacturers like Siemens. Look for “Surge current rating curves” in the MOV data sheets and compare rating and life for short versus long pulses. It is very clear.

Regards, Paul

You are absolutely right! Most SPDs do not incorporate fully coordinated safety controls. These devices contain blind spots within their fault current protection controls that will not clear during certain extended overvoltage or fault conditions. This means the SPD will short out the electrical system compromising safety and reliability.

This is not the case for APT SPDs. We’ve been manufacturing integrally mounted SPDs for Siemens Industry for nearly 14 years and until the early 2000′s Schneider Electric. Since our SPDs incorporated patented safety controls, the 10′s of thousands of SPDs they’ve mounted integral within their equipment have never compromised electrical system safety or reliability.

Check us out at http://www.aptsurge.com

Siemens Industry has private labeled our SPDs for the last 14 years, and they are integrally mounted within their electrical distribution equipment. Also, not long ago, Schneider Electric and American Power Conversion private labeled our integral and wall mounted style SPDs.

UL 1449 3rd Ed. Type 1 SPDs must include fully coordinated safety controls. Fault current protection can not be achieved through the use of external safety dis-connectors (i.e.. breakers or fuses). Since UL Type 1 SPDs are constructed in this manner, they can be directly connected to either the “line” or “load” side of the main service without the need of a fuse or breaker disconnect.

Siemens Industry, Schneider Electric, and APC private label or labeled our patented SPDs, and they connected them within their distribution equipment without the need to use a fuse or breaker. Since our SPDs incorporate fully coordinated safety controls, we’ve never had an incident compromising the integrity of electrical system.

If you want to learn more about UL 1449 3rd Edition, you can download our white paper by copying and pasting the following URL within your browser:

http://www.aptsurge.com/docs/UL3rdFlyer.pdf