Operation of current transformers

Exchanging a measuring device (short-circuiting of current transformers)

The current transformer secondary circuit should never be opened when current is flowing into the primary circuit.

The current transformer output constitutes a current source. With an increasing burden the output voltage therefore increases (according to the relationship U = R x I) until saturation is reached. Above saturation point the peak voltage continues to rise with increasing distortion, and attains its maximum value with an endless burden, i.e. open secondary terminals. With open transformers it is therefore possible that voltage peaks may arise, which could pose a risk of danger to persons and may also destroy measuring devices when reconnected.

It is therefore the case that open operation of CTs must be avoided and unloaded current transformers must be short circuited.

Current transformer terminal block with short circuit devices

In order to short circuit current transformers and for the purpose of recurrent comparative measurements it is recommended that special terminal block for DIN rails be used.These comprise a cross-disconnect terminal with measuring and test equipment, insulated bridges for grounding and short circuiting of the current transformer terminals.
Fig.: Current transformer terminal block

Overloading of measurement CTs

Primary current overloading:
Primary current too high --> Saturation of the core material --> Precision declines dramatically.

Nominal power overloading:
Too many measuring devices or excessively long lines are connected to a transformer with its defined nominal power --> Saturation of the core material --> Precision declines dramatically.

Instance of short circuit at CT secondary side

In the event of a short circuit no signal is available. It is not possible to measure with the measuring device. Current transformers can (or must) be short circuited if no load is present (measuring device).

Operation with harmonics

Our current transformers generally measure harmonics up to 2.5 kHz (50th harmonic) and many types also measure to 3 kHz and even beyond. However, with higher frequencies the eddy current losses increase and heating up is consequently also greater. If the total harmonic distortion is too high then the current transformer must be designed with thinner sheets.

However, it is not possible to make a general statement regarding a threshold value of the total harmonic distortion because heating up is dependent on core size, transformer surface (cooling), ambient temperature, ratio, etc.

Power requirement UMGs, energy meter, measuring devices

Measuring device type	Power consumption current measurement input in VA
Analogue ammeter	1,1
UMG 103 / 104 / 604 / 605	0,2
UMG 96RM	0,2
UMG 96RM-E	0,2
UMG 508 / 509	0,2
UMG 511 / 512	0,2
ECSEM series energy meter	0,36

Power consumption UMG 96RM-E per current measurement input


UMG 96RM-E	0,2 VA
	+
4 metre 2-wire line 2.5 mm²	1,43 VA
	=
Gives the power consumption of the measuring equipment the CT has to be rated for	1,84 VA

The special case: Larger current transformer – lower current

Tip:
Select a current transformer that is suitable for the measurement of a nominal current of 50 A.
In order to divide the normal current of a current transformer by two it is actually sufficient to run this current through the transformer twice.

Operation of current transformers