S+S Case Study

Achieve higher short circuit ratings for your control panel with the KT7 Motor Circuit Controller from Sprecher + Schuh.

As more emphasis is being placed on electrical worker safety around arc flash hazards from short circuits and ground faults, mitigating the thermal and mechanical hazards to workers and equipment with correct component selection and short circuit coordination is more important than ever. Industrial control panels designed to withstand the level of available fault current is a key aspect in reducing electrical hazards.   A common consideration for fault levels can be found in NFPA 70E.   Table 130.7(C)(9)references Hazard/Risk category 2*  for voltage testing of a 480 Volt control panel with available fault current of maximum 65 kA short circuit current available with a fault clearing time of 0.03 seconds(2 cycles) maximum.  Thermal and mechanical hazards are significant in a fault of this level as seen on the below video of a HRC 2*

Industrial control panels capable of withstanding these faults will limit excessive damage in the event of a short circuit.  Furthermore Sprecher + Schuh's KT7 circuit protection devices will significantly reduce the release of hazardous energy should an accidental short circuit occur down stream of the protective device. With Sprecher + Schuh's circuit protection and support from Trapp Industrial Sales, you can achieve higher short circuit ratings and improve worker safety. 

Short Circuit Current 

Short circuit current ratings refer to the level of damage allowed to equipment under a short circuit condition, and the speed of operation of the protective device is the key to current limitation.  With speeds approaching 2/1000 of a second (2 ms.), the Sprecher + Schuh KTA7 Self-protected Combination Motor Controller limits the current of the short circuit to a  lower level.  This current limiting allows for short circuit capacity of up to 65 kA in 480Y/277 Volt systems (not appropriate for 480Volt ?systems) protecting downstream equipment and reducing a potential hazard.

The result is that less energy is available to cause personnel injury and equipment damage.  Competitive devices typically have an operating time of around 6ms (green area) allowing significantly more let-through energy (I2T).   By choosing the correct components, you can increase the SCCR of your control panels and limit damaging short circuit energy from passing through.

Sprecher + Schuh makes it easy

The NEC and UL require contactors and overloads to be short circuit tested in combination with short circuit protection devices.  These ratings are published and define the maximum fault current that the tested combination can withstand without sustaining excessive damage (as defined by the criteria below).  This is the Short Circuit Current Rating (SCCR) of the specific combination of components tested.  Self-protected Combination motor controllers fulfill the functions of short-circuit protection, overload protection and approved disconnecting means in a manual controller which can be combined with a contactor and bears UL Approval.  The approvals are posted to the UL - SCCR ratings web site. Selecting the correct combination for your control panel not only requires the correct SCCR rating; it also requires the understanding of various levels of performance of the combination as defined by the UL.  Allowable damage sustained by the devices during a short circuit and the capability of a device to function after a short circuit is referenced by construction types.

Good, Better, Best

  • Type F Construction Type 1 - Combination Motor Controller
    • Damage to the Contactor or Overload is acceptable
    • Type 1 Short Circuit Coordination Performance
    • Branch Circuit Protection Device may be used as a Disconnect
  • Type F Construction with Type 2 Short Circuit Coordination Performance
    • No Damage to the Contactor or Overload is Allowed
    • Shall be suitable for future use
    • No Endurance standard after short-circuit
    • Manufacturer must Self-certify Testing
    • Branch Circuit Protection Device may be used as a Disconnect
  • Type E Construction - Self Protected Manual Combination Motor Controllers
    • Exceeds Type 2 Short Circuit Coordination Performance- No Damage After a Short Circuit Allowed
    • "Life after Short Circuit"- Must be usable in its existing condition
    • Endurance Standard for Performance after a Short Circuit Must Be Met
    • Branch Circuit Protection Device may be used as a Disconnect

Sprecher + Schuh publishes a convenient chart listing the combination of the KT7 device with applicable contactors and overloads (for three component systems) showing the resulting ratings for each of the above categories (see chart).  Building an industrial control panel for a specific available fault current has never been easier.

Example: Designing a UL Type E solution

Step 1 - Indentify the short circuit rating for power components: 

  • L11 Non-Fused Disconnect                        200 kA
  • Motor Circuit -       KTA7-45H-10A/CA7-30*   65 kA (see chart)
  • Motor Circuit -       KTA7-45H-45A/CA7-37*   65 kA (see chart)
  • Control Circuit -     KTU7-D-2D-5/CPT          100 kA
  • KT7Supply Block-  KT7-25-A3E                    65 kA
  • Connection with S+S UL Approved connection module

Step 2 - Determine the effect of current limiting devices in the feeder circuit

  • Not required to limit current to low rated devices

Step 3 - Determine the overall short circuit rating of the panel

  • Branch Ratings
          ? SCCR Branch #1   KTA7-45H-10A/CA7-30    65 kA (see chart)

          ? SCCR Branch #2   KTA7-45H-45A/CA7-37   65 kA  (see chart)

          ? SCCR Branch #3   KTU7-D-2D-5/CPT          100 kA        
  • Feeder Components
         ? L11 Non Fused Disconnect                             200 kA

         ? KT7Supply Block    KT7-25-A3E                       65 kA

         ? KT7 Bussbar           KT7-25-DB-45-2               65 kA
  • Overall Weakest Element                           65 kA

Overall Panel Rating 65 KA


Control Panels designed and built to withstand the available fault current in the feeder circuit are critical to mitigating hazards during downstream short circuits. With Sprecher + Schuh�s KT7 protection devices, industrial control panels can achieve SCCR levels up to 65 KA with Type E (life after short circuit) performance.  Furthermore, by reducing let-through current during a fault the Sprecher + Schuh KT7 will reduce damage to equipment and substantially reduce the hazard to workers in the event of an accidental short circuit.  For more information or specific application questions please contact Trapp Industrial Sales. 

Trapp Industrial Sales Engineers have the experience and the training to help you with all of your Sprecher + Schuh product line applications.

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