Current Transformers:
A Tester Survival Guide
Bryan Shannon
ABB Inc.
Coral Springs, FL
Current Transformer: Equivalent
Current is Stepped
Down [by primary and
secondary windings]
Exciting Impedance is
the primary source of
error. [By design Zo, Zi
or Xm is very high to
reduce Ie as much as
During Open CT
Condition. A very high
voltage is developed at
CT terminals. [all
secondary current flows
through Xm]
Why Relay Accuracy CTs?
Under short circuit conditions, a
small core that might be adequate
for metering applications would “fill
up” and be unable to carry the
magnetic flux required to transform
the current.
When this happens, the current
transformer might be unable to
induce a voltage high enough to
maintain proper current flow in the
secondary side. This could result in
very serious errors.
Protection CTs require linearity in a
wide range of currents
Calculated or Tested Accuracy
According to IEEE standards (C57.13), current transformers
are given an accuracy rating with a “C” or a “T”
“C” indicates calculated accuracy and “T” indicates tested
Using this system, current transformer with rating T100 would
have to be tested to verify that it could sustain a voltage of
100 volts within normal accuracy limits.
“C” type are transformers which are constructed so that the
effects of leakage fluxes on its performance are negligible,
such as bushing current transformers with uniformly
distributed windings.
Maximum Burden
Current transformers for relay applications are rated in the terms of
the maximum secondary volts that can be induced on a twenty
times short circuit rating – with error of current transformation
limited to 10%.
For example, a current transformer rated 5 amperes secondary
current, might be given a relay accuracy rating at 200 volts (C200).
This would indicate that the current transformer could sustain
relaying accuracy – at 100 amps – as long as the secondary voltage
IZ did not exceed 200 volts.
In this case, the maximum burden that could be used would be
calculated as follows:
Z (Burden)= E/I ( voltage rating / 20 times rated current of 5
= 200 volts / 100 amps = 2 ohms (Includes wiring and
Benefits of Test Switches
Test switches provide a quick and safe means of testing
Test switches are especially important wherever secondary
current transformer circuits may need to be temporarily
reconfigured to facilitate testing or where the relay must be
temporarily disconnected from service.
Accidental opening of a CT secondary circuit can result in
extremely high voltage and arcing, creating a dangerous
Test switches eliminate this possibility by diverting the
secondary current to an alternate path before opening the
connection to the relay.
This sequenced operation is inherent to the design of the
current-shorting poles of a test switch.
Circuit Design for Testing
Facilitate Simpler and Quicker Testing
•Allows Access to
Connections from
Front of panel
•Assures correct
testing procedure
sequence (Make
before break of
But, an Open CT can still occur.
Reasons for In-Service Testing
Competitive Electric Utility Market
More power wheeling/power needs
Control of supply chain resources
Requires reliable power delivery
Equipment availability
Deregulation of Electric Power
– GENCO to TRANSCO separation
– ISO activity requires metering
– Need to use existing ITs
Bottom-line Focused
– Billing and current swings
– Must verify performance of ITs
In Service Measurements – What could
go wrong
Accidental Opening of current
transformer due to:
-Blown Fuse
-Accidentally disconnected
-Defective leads
-Defective equipment
-Incorrect Connections
-Incorrect meter mode
Open CT
•Current goes to
zero until
condition is
•Every half cycle
high voltage
transients occur
Open Circuit Voltages
Because the open circuit voltage is limited by saturation of the
core, the RMS value measured by a voltmeter may not appear
to be dangerous. As the current cyclically passes through
zero, the rate of change of flux at current zero is not limited
by saturation, and is very high indeed. This induces extremely
high peaks or pulses of voltage. Voltage transients up to 15Kv
could be possible.
These high peaks of voltage may not register on the
conventional voltmeter, but they can break down insulation
and are dangerous to personnel. Current transformers are
insulated to withstand, for emergency operation, secondary
peak voltages up to 3500 volts
Current Dangers
Human Reaction (at 60Hz)
Current (milliamperes)
Perception Threshold
1 mA
Painful Sensation
3-10 mA
Can’t Let GO (paralysis or arms)
10 mA
Can’t Breathe (Paralysis of chest
30 mA
Fibrillation Threshold (Affects
Heart- Could be fatal)
65 mA
Heart Paralysis
Tissue Burning.
A value of 1500 ohms is commonly used as the resistance from arm to arm of
the human body.
What Level is a Dangerous Voltage?
Human Reaction (at 60Hz)
Voltage Required (Arm to Arm)
Perception Threshold
1 mA
Painful Sensation
3-10 mA
Can’t Let GO (paralysis or arms)
10 mA
Can’t Breathe (Paralysis of chest
30 mA
Fibrillation Threshold (Affects
Heart- Could be fatal)
65 mA
Heart Paralysis
Tissue Burning.
OSHA says 50Vac is a hazard [1910.333(a)(1) and 1910.269(I)(1)]
IEC (International Electrotechnical Commision): 30Vac Rms, 42Vac Peak, or 60Vdc
Consequences of Open CT
High voltage and shock hazard
Signal discontinuity to relay and
controls could lead to incorrect
operations and outages.
Potential magnetization of the
CT leads to incorrect secondary
output. This can cause error in
readings, revenue metering
calculations, incorrect operations
long after the open CT condition
is resolved.
Typical In-Service Testing
Test Switch
Solution to existing problems
Prevents Shock
Hazards [Created by
human error, and
other causes]
Test Switch
Prevent Blackouts
[Created by human
error, and other
Prevents CT
erroneous reading
(Avoid Magnetization)
Test Plug with Open CT Protection
How it works?
•Detects rate of
change of voltage
and internal circuitry
shorts CT
Test Switch
•LED would indicate
open CT condition
•Maintains signal to
•Eliminates possibility
of false trip
Test Plug with Open CT Protection
Open CT Protection
•Current signal
integrity to relay
maintained. [Prevents
incorrect operations]
•Voltage spikes
limited to first
•Voltage spikes
limited to 35 Volts
[Eliminates Shock
Current waveform in Open CT with New
Test Plug
distortion or signal
discontinuity to
protective devices,
eliminating false
trips and potential
User Exposure Comparison
Maintains voltage to safe levels and reduces user exposure, thus
increasing safety and eliminating equipment damage
Open CT conditions are a very likely event as more
in-service testing is required
Open CT condition leads to Shock Hazard, Outages
Open CT could lead to incorrect readings long after
condition is eliminated due to magnetization
Test Switches are recommended for installation with
Current Transformers
Proposed test plug device eliminate risks due to
operator error, incorrect equipment or risks arising
due to normal testing practices and procedures.