Chapter Corner

Service Entrance and Short Circuit Currents

Posted in: November/December 2015

Service entrance equipment can present unique challenges when it comes to proper application of electrical equipment. The service point is the point of demarcation between utility and premise wiring. Our discussion today will springboard off of a previous article that focused on calculating short-circuit currents. Let’s focus on the proper application of the equipment at the service entrance location in the power distribution system. We’ll reference the National Electric Code® (NEC) but with an understanding that this is also the point of demarcation between the National Electrical Safety Code® (NESC) and the NEC. Regardless of the code jurisdiction under which the application falls, when you drop back to the basics of electrical principles, we gain an understanding to help in the proper application of electrical equipment at this and any other location in the power distribution system. Attention to detail is warranted for safety.

safety.gifSERVICE EQUIPMENT

A service consists of conductors and other equipment essential for delivering power from the utility to the premise. The NEC provides the requirements for services in Article 230, titled the same. Although not always the case, the point of common coupling with the utility is typically the point most likely to present the highest level of short circuit current in the facility. This equipment must be adequate for the installation in both rating and listing. The ratings we’ll focus on today include Short Circuit Current Rating (SCCR) and Interrupting Rating (IR).

SHORT CIRCUIT CURRENT RATING

If you confuse SCCR with IR, you are not alone. I have heard these terms abused. To understand SCCR, consider it the ability of a piece of equipment to hold together while fault current flows through it on its journey to the downstream faulted portion of the circuit. The equipment has to be able to let this fault current pass through without damaging itself.

The equipment between the utility transformer and the faulted portion of the circuit must let the fault current pass through without experiencing an unintended rapid disassembly. The performance of the distribution equipment will depend upon many factors, one of which is the magnitude of fault current and the other is, of course, the capability of the equipment installed. The meter socket, for example, is not designed to stop the flow of current but rather serve its primary function of measuring the amount of energy being delivered to the facility. It must also serve its secondary function of delivering current to the load. The current it delivers is normally a load current that should not exceed its continuous current rating. The current could also be a temporary overload that should not exceed its maximum amp rating. Then again, the current could be a very high short-circuit current on the order of many times the magnitude of the normal load current, which should not exceed its SCCR. The meter socket will be equipped with an SCCR, which tells us how much current it can let through safely without causing damage to itself. Of course, there are many other types of equipment that may be located at the service entrance location, including power distribution blocks, busway, conductors, and more. The possible configurations are many but the basic principles remain the same, and each component that must deliver current to the load must be appropriately rated. They must have an adequate IR and SCCR.

Once we have the basic understanding of what an SCCR label means for electrical equipment and why it is important, it should be evident that all of the equipment that delivers normal load currents may be called upon to deliver high short-circuit currents and will need to have an adequate SCCR rating to do so without creating a hazard. Short-circuit currents place extreme forces on the electrical equipment in the distribution system that it passes through. That includes equipment and conductors. Any equipment that carries current will only do so safely when the magnitude of the current is less than its labeled SCCR. A review of the power distribution system single-line and/or three-line diagrams can help us understand the specific equipment in the system to which an SCCR rating must be applied.

As you read the labels of equipment, such as a meter socket, you will find that SCCRs upwards of 100kA can be achieved, but only when Over Current Protective Devices (OCPD), such as class J or class T, fuses of any ampacity not exceeding a specified amperage is provided upstream. Without an OCPD upstream, equipment such as meter sockets can only achieve an SCCR of 10kA.

Let’s explore how electrical equipment achieves higher SCCR ratings by exploring the basics of current limitation. The OCPD upstream ensures that the electrical equipment will not see the stresses and forces that high fault currents cause as the OCPD acts to interrupt the fault current faster than the blink of an eye. When properly protected by an upstream OCPD, the full level of fault current is never seen by the downstream protected equipment. Figure 1 shows the difference between what the equipment sees with and without the OCPD upstream.

Section 110.10 of NEC 2014 requires that the equipment SCCR “be selected and coordinated to permit the circuit protective devices to clear a fault, and to do so without extensive damage to the electrical equipment of the circuit.” Notice that Section 110.10 indicates that a specific circuit protective device (fuse or circuit breaker) might be required to provide proper protection. Section 110.10 also says that the protective device must protect the equipment from extensive damage, implying that damage can occur to equipment after a fault but it can’t result in a shock or fire hazard outside of the enclosure.

But let’s put the NEC aside for a moment and put on your electrical system basics hat for a moment. Regardless of what the Code requires, we need to design and install a system that is safe and will be safe for years to come through ensuring products are applied within their rating. When we understand the basic principles of how the equipment is designed and expected to be used, the proper application will follow.

We’ll leave this discussion with the definition of SCCR from the NEC: “The prospective symmetrical fault current at a nominal voltage to which an apparatus or system is able to be connected without sustaining damage exceeding defined acceptance criteria.”

INTERRUPTING RATING

The other rating that deserves attention is the IR of the OCPD. The difference between a device that has an IR and one that has an SCCR is that the former device acts to stop the flow of current and the other does not. IR is defined in NEC 2014 Article 100 as “the highest current at rated voltage that a device is identified to interrupt under standard test conditions.” The IR of an OCPD is the highest current that an OCPD is rated to safely clear. According to NEC Section 110.9, the IR must be greater than the maximum available short-circuit current available at the equipment’s line terminals.

The NEC requires the marking of the IR of fuses per Section 240.60(C) and circuit breakers per Section 240.83(C). The IR of these devices will depend upon their voltage rating as well as their functioning in AC or DC systems. A single circuit breaker will have varying IRs dependent upon the voltage at which it is applied.

If a violation of Section 110.9 or Section 110.10 occurs and the fault current exceeds the IR of the OCPD or SCCR of equipment, a catastrophic and violent failure of the OCPD or equipment can occur.

CHALLENGES AND RISK

The unique challenge presented at the service entrance equipment is the fact that often there is no OCPD upstream of the service equipment. The following text is paraphrased from a utility customer guide for electric services.

“All metering shall be installed on the line- side of the customer’s main disconnecting means except for a 480Y/277 volt metering installation, where the Company requires a non-fusible meter disconnect ahead of the meter or where the NEC requires a main disconnect ahead of a group of more than six (6) meters.”

A non-fused meter disconnect ahead of the meter has an SCCR of 10kA. This would be adequate only if the available fault current at that location is less than 10kA. There are some jurisdictions that require the meter disconnect located ahead of the meter to be a fused disconnect. This unfortunately is not the norm.

Equipment with insufficient SCCR subjected to a short-circuit event can expose personnel and equipment to serious danger. Without sufficient SCCR, it is likely that the equipment will sustain and cause damage inside and outside of the equipment.

Insufficient assembly SCCR poses the following hazards:

  • Electric shock and burns;
  • Burns associated with arc flash and contact with heated surfaces;
  • Injury associate with flying debris;
  • Damage to equipment or the facility;
  • Arc blast (shock waves, shrapnel, etc.); and
  • Vaporized metal.

It is very important that all electrical equipment be applied within its rating. SCCR is one such rating that when exceeded can be quite hazardous to those in and around the equipment.

As always, keep safety at the top of your list and ensure you and those around you live to see another day.

Thomas Domitrovich, P.E., manages a team of Application Engineers for the Circuit Protection Division of Eaton Corporation within the Bussmann business, an IEC National Platinum Partner. Thomas is based out of St. Louis, Missouri, and has more than 25 years of experience as an Electrical Engineer. He is a LEED Accredited Professional and a licensed Professional Engineer in the state of Pennsylvania. Thomas is active in various trade organizations including the Independent Electrical Contractors (IEC), International Association of Electrical Inspectors (IAEI), Institute of Electrical and Electronic Engineers (IEEE), National Electrical Manufacturers Association (NEMA), and the National Fire Protection Association (NFPA). Thomas is Principle member of Code Making Panel 2 for the National Electrical Code (NFPA70) and an Alternate member on NFPA 73 both, representing NEMA.