Topic: Safety Systems

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Busting Myths About Foundation Fieldbus
Mythperceptions About Fieldbus Can Be Changed

An Update on Liptak and Shinskey
Greg Shinskey Continues to Do Consulting and Bela Liptak Talks on Optimization and Safety

Really? A Political Diatribe?
Reader Disagrees with Editor in Chief Walt Boyes's View on External Cyber Attacks Dangers and U.S. Cybersecurity Policies

Liar, Liar, Pants on Fire?
Our Very Own Ask the Experts Moderator Bela Liptak Learns a Lesson From One of Our Readers. Liptak Admits That Even Knowledgable People Can Be Wrong

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White Papers: In Depth Research

Permanent Electrical Safety Devices Will Verify Zero Electrical Energy
Author: Grace Engineered Products, Inc.
Posted: 10/20/2011
Article 120.1 of the NFPA 70E establishes the procedure for creating an electrically safe work condition. Since this was written, the day-to-day practice of electrical safety has changed going beyond the precise language of Article 120.1(1-6). This is due to the increased usage of permanent electrical safety devices (PESDs) in Lock-out/Tagout procedures. The relatively new concept of permanent electrical safety devices actually improves the workers' ability to safely isolate electrical energy beyond that which was originally conceived when Article 120 was written. PESDs go beyond the high standard, yet they still adhere to the core principles found in Article 120.1. With PESDs incorporated into safety procedures and installed correctly into electrical enclosures, workers can transition the once-risky endeavors of verifying voltage into a less precarious undertaking that never exposes them to voltage. Since, every electrical incident has one required ingredient - voltage, electrical safety is radically improved by eliminating exposure to voltage while still validating zero energy from outside the panel.

Is There a One Size Fits All SPD?
Author: Mersen
Posted: 05/24/2011
Frequently, our customers will ask for a "one size fits all" Surge Protective Device (SPD), eliminating the need to stock several different part numbers to meet their customers needs. Some manufactures claim to have a one size fits all Surge Protection Device (SPD), however there is absolutely no benefit of this to the end user. Why? The one size fits all approach could in most cases actually cause damage to the equipment it should be protecting.

For example, you would not want to use a 2000A fuse as a one size fits all fuse. If you only need a 30A fuse to protect the load, clearly a 2000A fuse would be significantly over sized. In the end, the fuse would never open, making it a poor investment for the facility. Now what if a 60A fuse was used instead? In this case, the fuse might open but when it does it will not protect the load as well as a properly sized 30A fuse.

This concept is the same for an SPD. While it is possible to have a "one size fits all," it is not recommended because in most cases it will not provide adequate protection. To help make this point let's look at some part numbers and compare the ratings. To ensure our "one size fits all" SPD can be applied to all common voltage configurations we must use a 480V delta SPD, part number STT24803PDG.

This configuration could be installed on a 208Y/120V system but how much protection will it provide? Answering this question involves reviewing the MCOV and VPR ratings. MCOV is the turn on voltage of the SPD and is where it will start to clamp the excess voltage. From the table below, STT24803PDG has an MCOV of 550V line to ground which is about 450% of our circuit line to ground voltage. Typically the MCOV should be between 125% to 150% of the voltage.

What Is the Surge-Trap SPD?
Author: Mersen
Posted: 05/24/2011
The Surge-Trap is a branded surge protection device (SPD)that utilizes Mersen's patented thermally protected metal oxide varistor (TPMOV) technology. This technology eliminates the need for fuses to be installed in series with the Surge-Trap SPD.

which saves money and panel space. Surge-Trap SPD is typically installed in industrial control panels to protect sensitive electrical equipment from harmful voltage transients. Nearly 80% of all transients are caused by equipment or power disturbances within a facility.

What Types of Ratings Do SPDs Have?
Do SPDs have a current rating? This is a trick question! They do not have a continuous current rating however they do have other important current-based ratings. They are required to have a short circuit current rating (SCCR), which is the maximum rms current at a specified voltage the SPD can withstand.

The nominal discharge current (In) is new to UL 1449 Third Edition (effective 9/29/09). This is the peak value of the current (20kA maximum) through the SPD (8/20μs waveform) where the SPD remains functional after 15 surges.

There are two main voltage ratings for an SPD, the first is maximum continuous operating voltage (MCOV) which is the maximum rms voltage that may be applied to the SPD per each connected mode.

Voltage protection rating (VPR) is determined as the nearest high value (from a list of preferred values) to the measured limiting voltage determined during the transient-voltage surge suppression test using the combination wave generator at a setting of 6kV, 3kA.

How Do I Select The Correct SPD?

Understanding Ratings for Surge Protection Devices
Author: Mersen
Posted: 05/24/2011

Selecting the appropriate Surge Protective Devices (SPD) can seem like a daunting task with all of the different types on the market today. The surge rating or kA rating of an SPD is one of the most misunderstood ratings. Customers commonly ask for an SPD to protect their 200A panel and there is a tendency to think that the larger the panel, the larger the kA device rating needs to be for protection As we will explore in this paper, this is a common misunderstanding.

When a surge enters a panel, it does not care or know the size of the panel. So how do you know if you should use a 50kA, 100kA or 200kA SPD? Realistically, the largest surge that can enter a building's wiring is 10kA, as explained in the IEEE C62.41 standard. So why would you ever need a SPD rated for 200kA? Simply stated - for longevity.

So one may think: if 200kA is good, then 600kA must be three times better, right? Not necessarily. At some point, the rating diminishes its return, only adding extra cost and no substantial benefit. Since most SPDs on the market use a metal oxide varistor (MOV) as the main limiting device, we can explore how/why higher kA ratings are achieved. If an MOV is rated for 10kA and sees a 10kA surge, it would use 100% of its capacity. This can be viewed somewhat like a gas tank, where the surge will degrade the MOV a little bit (no longer is it 100% full). Now if the SPD has two 10kA MOVs in parallel, it would be rated for 20kA. Theoretically, the MOVs will evenly split the 10kA surge, so each would take 5kA. In this case, each MOV have only used 50% of their capacity which degrades the MOV much less (leaving more left in the tank for future surges).

Does this translate into surge "stopping power?" No, just because an SPD has 2 or 20 MOVs in parallel it does not mean it will limit the 10kA surge any better then a single SPD (of the same rating). The main objective of having MOVs in parallel is to increase the longevity of the SPD. Again, keep in mind that it is subjective and at some point you are only adding cost by incorporating more MOV’s and receiving little benefit.

As mentioned before, panel size does not really play a role in the selection of a kA rating. The location of the panel within the facility is much more important. IEEE C62.41.2 defines the types of expected surges within a facility as:
Category C: Service Entrance, more severe environment: 10kV, 10kA surge
Category B: Downstream, greater than 30' from category C, less severe environment: 6kV, 3kA surge
Category A: Further downstream, greater than 60' from category C, least severe environment: 6kV, 0.5kA surge

How do you know what kA rating to use? The IEEE categories provide a good base for selecting kA ratings. There are many "right" sizes for each category but there needs to be a balance between redundancy and added cost. Qualified judgment should always be used when selecting the appropriate kA rating for an SPD.

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