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What is a Short Circuit and or Relay Coordination Study?

Article by: cpswitchgear.com

Why specify a short circuit and relay study?

Does your facility use trip units (relays) on circuit breakers? Have the settings been adjusted by one of the plant electricians so nuisance trips on the breaker was stopped? Do you know what the existing device settings are, and are they correct? Too many facilities are being protected by trip units that are not set for the correct operation of the circuit breaker. The main breaker to the switchgear may be tripping before the load breaker that supply’s a motor trips. The most common reason for the incorrect circuit breaker coordination tripping sequence is because the relays are not set as designed for the switchgear.

What is the definition of a short circuit and or relay coordination study?
To define a short circuit and or relay coordination study in simple terms, coordination means that downstream devices (breakers) should trip before upstream breakers. This keeps the fault at or near the equipment that causes the inrush of current to occur. Short Circuit and Coordination Study’s are designed not only to protect the equipment, but to save lives. The correct interrupting rating is critical for safe, economical, and efficient operation of any electrical distribution system. If you have seen an electrical fault where current is greater than the interrupting rating of the protective apparatus, the cost, and consequences can be serious. Personnel injury, damaged equipment and downtime are all too common when the relay coordination is not set to correctly open circuits when called upon. Coordination Studies isolate the first protective device inline to the fault.

Below is the beginning of a relay coordination study Carolina Precision Switchgear provided for one of our customers.

Information used was provided via a power riser diagram E0.2 and copy of hand written notes of existing settings and breaker trip unit sizes. Where information was lacking it is note below.

Cable information was provided or derived from the power riser diagram.

The TE1L1 and TE1L2 transformers are protected at 175% for Long Time and Instantaneous set 10 times the full load amps. The Short Time is set to allow inrush without nuisance tripping. Both breakers also coordinate with DB-1.

This also applies to the transformers feed from breakers E1L3 and E1L4.
E1HE1, E1HL, UPS, E1HM1, E1HM2, UPS2, and UPS BYPASS breakers are set to protect the cable and coordinate with DB-1.

The DB-1 breaker is set to coordinate with all the above breakers and is set slightly below the Main breaker MSB-2

The MSB-2 breaker is set to coordinate with all sub-feed breakers and protect the cable feeding the DB-1 Switchboard.

You have a full investment in your equipment, so why use it to its full and capable potential?

Don't wait for an outage to find out there's a problem.

Contact Carolina Precision Switchgear today at www.cpswitchgear or phone 704-596-4242 to let us help you with your nuisance trips, and help fully protect your valuable personnel and equipment.


Digitrip 1150 Rating Plug

Article by: cpswitchgear.com

The following is the process to change out the overcurrent rating plug in an Eaton/Cutler Hammer Digitrip 1150 and 520 family of Power Circuit Breaker Trip Units. I will explain below the process, but first I want to make sure you remember a few basic steps before inserting a new rating plug. I will also talk a little about the operation of the current transformers.

Let's say for instance the tap is rated at 600 amps. If the circuit breaker is set up properly, the current transformers ( CT's )on each phase will also be 600 amps. Always check the current transformers before changing and inserting a new tap into the relay. A 1200 amp tap will fit into the same relay, but the breaker will not trip on the overcurrent settings as designed. ***** Never energize any circuit breaker without the overcurrent device attached. Damage to the internal CT's may occur because the circuit is now open, making for no protection for the load circuit.

Now to insert the rating plug, push it into the cavity on the right-hand side of the trip unit. There are three pins that need to be aligned on the plug with the sockets in the opening. Insert the plug with minimal force and the rating plug is now inserted and ready for testing. The overcurrent relay can be tested either by primary, or secondary injection. The power relay module is a standard accessory that will power up trip unit module for testing purposes.

For more information, please visit www.cpswitchgear.com or call Carolina Precision Switchgear at 704-596-4242.