NEC 110.23: Current Transformers

Or: "How to Not Turn a CT Into a Tesla Coil of Death"

The Plain-Language Breakdown

Alright, gather 'round. Let's talk about current transformers (CTs) and why leaving one open-circuited is like leaving a rattlesnake in someone's lunchbox – it's gonna bite somebody, and it ain't gonna be pretty.

Here's the deal: If you've got a current transformer hooked up to an energized circuit but you're NOT using it (maybe you disconnected the meter, or you're troubleshooting, or Kevin from day shift "temporarily" pulled the wires six months ago), you gotta short-circuit that sucker. And I mean now. Not "I'll get to it after lunch." NOW.

Why? Because a CT with an open secondary is like a teenager with a credit card – dangerously uncontrolled. These things can generate voltages in the thousands of volts range when there's no load on the secondary. We're talking arc-flash city, fried equipment, and potentially becoming a hood ornament on the safety stand-down PowerPoint presentation. Nobody wants to be that guy.

The rule is dead simple: CT connected to a live circuit but not being used? Short the secondary terminals together. Most CTs even come with a shorting block for exactly this reason. It's usually yellow or has "SHORT HERE" printed on it, because apparently we needed that reminder after Gary from '97 learned this lesson the hard way.

Key Takeaways

🔧 CRITICAL TECHNICAL POINTS:

• Unused CTs on energized circuits MUST be short-circuited – This is not a suggestion or a "nice-to-have"
• "Unused" means: The secondary isn't connected to a meter, relay, or other load
• "Energized circuit" means: Current is flowing through the primary (the big conductor going through the CT donut)
• How to short it: Connect the secondary terminals together, typically using the manufacturer's shorting terminal or a wire jumper
• This applies even during temporary conditions – Testing, troubleshooting, whatever. No exceptions
• Open-circuit voltage danger: An open CT secondary can generate voltages exceeding 10,000V (yes, you read that right)
• Insulation breakdown risk: Those lethal voltages can destroy the CT's insulation and create shock/arc-flash hazards

Real-World Jobsite Scenarios

Scenario 1: "The Meter Swap Gone Wrong"

You're replacing a revenue meter on a 480V switchgear lineup. You pull the meter out to swap it, and head to the truck to grab the new one. Your apprentice (bless his heart) sees those CT secondary wires just hanging there and thinks "I should tape these up so they don't touch anything."

WRONG MOVE, SPARKY.

The primary conductors are still hot, carrying hundreds of amps. Those innocent-looking 18 AWG wires on the secondary can now be sitting at several thousand volts. When you come back and grab them barehanded... well, let's just say you'll remember this Code section REAL well after that.

THE RIGHT WAY: Before pulling that meter, land those CT secondary wires on the shorting block. It's literally right there on the meter socket. Takes two seconds.

Scenario 2: "The Relay Upgrade Surprise"

You're upgrading protective relaying in an industrial facility. The engineer spec'd new multifunction relays to replace the old electromechanical units. You disconnect the old relay to pull it out, planning to wire in the new one "right after lunch."

Plot twist: The plant manager needs that switchgear energized for production. Now you've got CTs with open secondaries on a live 4000A bus. The voltage buildup starts cooking the CT insulation, and when you come back to wire in that new relay, you get hit with an arc flash from the degraded insulation.

THE RIGHT WAY: Install shorting terminal blocks BEFORE you disconnect anything. When you pull that old relay, the CTs are already protected. Wire in the new relay, then remove the shorts. Work smart, not hurt.

Scenario 3: "The Troubleshooting Trap"

You're hunting down a metering problem. You disconnect the CT secondary wires from the meter to check for voltage (classic troubleshooting move). You get your reading, set your meter down, and start looking at the panel schedule to figure out the circuit.

Five minutes later, you lean across the gear and your arm brushes those disconnected CT wires. ZAP.

Congratulations, you just learned about induced voltage the hard way. Your FitBit thinks you ran a marathon, but you only traveled three feet – straight up.

THE RIGHT WAY: When troubleshooting, use a shorting block or jumper wire. Check your voltage, get your readings, then immediately short those terminals before you do ANYTHING else. Treat those CT secondaries like they're trying to kill you, because they literally are.

What to Study (For the Test and Your Life)

HIGH-YIELD EXAM MATERIAL:

1. The basic rule: Unused CTs on energized circuits = must be shorted

   • They LOVE asking this on masters exams
   • Question format: "A current transformer is temporarily disconnected from its associated meter while the primary remains energized. What is required?"

2. What "unused" means:

   • Secondary not connected to a load (meter, relay, instrument)
   • Primary still energized (current flowing)
   • Both conditions must exist

3. Why this rule exists:

   • Open CT secondary can generate dangerously high voltages
   • Voltage is induced proportional to the primary current
   • Can exceed 10,000V easily on higher-current installations

4. Common test distractors to watch for:

   • "De-energize the primary" – NOT what the Code requires (though it's safer if you can)
   • "Disconnect both primary and secondary" – Nope, just short the secondary
   • "Install a grounding conductor" – NO! That's not the same as shorting the terminals together
   • "Add additional insulation" – Wrong answer, pal

5. Practical application knowledge:

   • Know where the shorting terminals are located (usually on the meter socket or junction box)
   • Understand this applies during testing, maintenance, and troubleshooting
   • Remember: TEMPORARY conditions still require compliance

MEMORIZATION TIP: "CT not in use? Short the deuce!" (The "deuce" being the secondary, because it's the second winding... yeah, I'm reaching, but you'll remember it.)

SAFETY REALITY CHECK: This rule has a body count. There are actual case studies of electricians and techs getting nailed by open CT secondaries. The voltage doesn't care about your 20 years of experience or how careful you think you're being. Physics is physics, and induced voltage is a stone-cold killer.

The Bottom Line

This is one of those Code sections that's short and sweet but backed by real carnage in the field. Current transformers are awesome devices – they let us measure big currents with small wires – but they're based on transformer action, and transformers can step voltage UP just as easily as down.

An open secondary CT is basically a step-UP transformer with no limits. The primary might be at 480V with 400A flowing through, but that secondary can spike to voltages that'll blow right through your insulation, your gloves, and possibly your life insurance policy.

The fix is stupid-simple: Wire nut those secondary leads together, or use the shorting block that's already there. It takes literally five seconds and costs nothing.

So next time you're pulling a meter, swapping a relay, or troubleshooting a CT circuit, remember: Short first, work second, go home with all your fingers third.

Stay safe out there, and for the love of Mike Holt, short those CTs.

Remember: The NEC is the bare minimum. Your state/local codes might have additional requirements, and your company safety policy better be even stricter. When in doubt, short it out (the CT, not the feeder – let's not get crazy).