CTs are used for the measurement of high currents as a
primary sensor. The output of the CT is normally connected
to the relevant ammeter which has very low impendence and
secondary remains almost short circuited. The secondary
current has no control over primary current in a CT. If
secondary is open circuited two things will happen :
1. Due to absence of secondary current no counter magnetic
field is produced and flux in core of CT rises to
saturation level which can change the charateristics of CT
and damage it permanently
2. since number of turns in secondary is very high (1:
1000) for , a very high voltage may be induced in secondary
that can harm the prsons working on the instruments panel.
if secondary is open then secondary current and flux will be zero. so that total flux is increased.
emf is directly proportional to flux than there will be induced a high voltage that saturate the core and demage the transformer.........thus we should not be open secondary .
CTs are used for in the measurement of current as a primary
sensor. The output of the CT is connected to the relevant
power meter. For CT the primary is the busbar / cable
caarying current which generates current in the secondary
by induction. Now if CT secondary is kept open and current
flows through primary the induced emf in the secondary will
not have a close path to flow which will lead to opening /
bursting of the CT.
The CT secondary should not be left opened secondary due to
1)Zero current in secondary leads zero back emf.So primary
current becomes very high as the primary resistance is very
low.Large heat generated may burn the winding and core.If
the transformer sustain this heat, even then its operation
becomes improper as explained below.
2)CT is bascially a step up transformer.So high voltage
induced in secondary may saturate the core (core becomes
permanently damaged).So the operating point advances in non
linear region,which makes the transformer operation improper
(secomdary current will not replicate the primary according
to the turn ratio).
3)High voltage spikes(order of kV) produced in secondary side
may endanger the life of personnel working on the secondary
4)High voltage spikes(order of kV) produced in secondary side
is sufficient to break down the transformer insulation.
A current transformer should never be open-circuited while
main current is passing through the primary winding. If the
load is removed from the secondary winding while the main
circuit current is flowing, most of the primary winding
current becomes magnetizing current, but the vector angles
change in such a way as to keep the total current in the
primary the same as before. Because the main circuit is now
mostly magnetizing current, the flux in the core shoots up
to a high level and a very high voltage appears across the
secondary. Due to the high turn ratio usually found in these
transformers, the voltage in this condition can reach a
dangerously high level, which can break down the insulation.
It also becomes a hazard to personnel. The high flux can
saturate the core and result in strong residual magnetism
left in the core, thereby increasing magnetization current
and introducing error in the transformation ratio. One has
to put a short on the secondary winding before removing the
In the physical construction of CTs you can see the secondary
is made up of a winding but the secondary is just one cable or
busbar.So it acts as a step up transtrofmer when the secondary
is left open.It is real I have seen two people who have had their hands burnt due to careless handling of the CTs.
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