You’re not as safe as you think you are
With today’s advances in engineering and safety, why is it one maintenance person is either killed or injured each day in an electrically-related incident?
In my opinion, it is possible because the large majority of effort that has gone into engineering and inspecting for safe electrical systems may end when the electricity reaches the line side terminals of that new piece of equipment installed in your plant. This oversight has now been corrected in the 2005 National Electric Code, published by the National Fire Protection Association (NFPA). Now, labels are required on equipment that clearly state the equipment’s Short Circuit Current Rating (SCCR). The NEC specifically addresses this for industrial control panels (Article 409), industrial machinery electrical panels (670), multiple motor HVAC equipment (440), meter disconnect switches (230) and multiple motor controllers (430). I believe that the standard should also apply to all commercial and industrial equipment.
This is a huge leap forward. But I believe that the industry needs to be better educated on the SCCR of equipment and the danger associated with not understanding it.
Equipment that you install in your plant must be either UL Listed to 508A (after April 25, 2006) or have witnessed and documented SCCR testing results. This equipment must also be properly labeled with this SCCR rating. In addition, your facility fault current studies must be up to date and accurate. If you have met these requirements, your only concern is that the labeled SCCR of the equipment is greater than the available fault current at the line terminals of that equipment.
Very few electrical equipment manufacturers are producing equipment today that meets these SCCR requirements.
The most dangerous and common misconception of SCCR by equipment manufacturers is that the interrupting capacity or rating of a circuit protection device located in or protecting the equipment is also the SCCR of that equipment when it is installed. In other words, for example, sometimes the manufacturer that labels the equipment with a 22 kA SCCR, does so solely because the main circuit breaker or fuse has an interrupting capacity rating of 22 kA. This is a mislabeling of the equipment and creates a potentially dangerous and code-violating condition in your plant.
In order to build and label a safe piece of equipment, the equipment manufacturer must determine which component in the primary electrical path has the lowest SCCR or withstand rating. For example, suppose a commonly used IEC definite-purpose contactor has a withstand rating of 5 kA. With only one exception, the panel in which this component is installed must be labeled with a 5 kA SCCR, and that equipment then must be installed at a location in your plant with less than 5 kA of available fault current. That exception is, if the low withstand-rated component is protected with a current-limiting fuse or circuit breaker that is engineered to reduce the greater-than-5 kA fault current to a level less than 5 kA RMS. Be aware: Not all breakers and fuses are current limiting. If they are, they will be labeled and UL Listed as such.
Why is this so important? Just as every device within the electrical distribution system of your facility must be rated to handle a worst-case scenario in order to completely protect the people and equipment within your facility, every component within your equipment must be designed to handle a worst-case scenario for exactly the same reason. Seems obvious, doesn’t it?
I have a challenge for you. Think of your most recently installed major piece of equipment. Find your most recent fault current study and look up what the available fault current is at that point in your electrical distribution system. If you don’t have a recent fault current study, get one done (you’ll need it for your arc flash hazard study for OSHA anyhow), or go to the first transformer on the line side of the equipment.
For a rough worst-case estimate, divide the kVA by the voltage, divide that by the labeled impedance, and then multiply that by 1,000. Now, go to the equipment. Find the label on the equipment with the SCCR, if there is one, and compare it to the available fault current. If the SCCR is greater than the available fault current, good — but you’re not done yet.
Now either de-energize the panel or put on your protective equipment and open the electrical enclosure door. Find the interrupting capacity of the main fuse or breaker. If the SCCR of the equipment is equal to the interrupting capacity of that device, you need to start getting suspicious of the validity of your equipment’s SCCR.
Final step: look for the component in the main electrical path that you feel would have the lowest short circuit withstand rating. Now, follow the line side back to the main fuse or breaker. If you do not come across a current limiting device in this path, you also need to be suspicious of the validity of the SCCR of that equipment.
A majority of you just realized that you have a problem.
If you have a short circuit condition, the breaker or fuse is going to open safely, right? Yes, but the problem isn’t with the breaker or fuse operating properly; the problem is, does it operate quickly enough? If a component in a piece of equipment has a withstand rating lower than the available short circuit current, it is going to fail catastrophically under a short circuit condition. Nobody wants to put their people in front of a piece of equipment that they know can harm them. And now we’re back to the National Electric Code change.
After years of assuming that enough parts of the NEC specifically address overcurrent protection such that most of the dangerous misapplications of underprotected low withstand rating devices shouldn’t exist, the NEC finally recognizes and specifically requires equipment to have accurate SCCR labeled. These labels will now allow you and inspectors to compare fault current studies to the equipment SCCR and ultimately and effectively eliminate the dangerous applications of equipment in your facility. Until equipment manufacturers are educated and proactively change their circuit protection, inspectors are taking this one step further, and not just trusting the label.
I suggest that you do the same.
|Dan Gilman is the OEM Sales Manager of the POWR-GARD Electrical Business Unit of Littelfuse and has held this position since 2004, previously serving as the Regional Sales Manager for the South Central United States. For questions about SCCR, or what the NEC has to say about it, contact Mr. Gilman directly at dgilman@Littelfuse.com .|