Delta MCS-1800 Specifications download pdf (Page 63)

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DETERMINING BURDEN FOR CURRENT TRANSFORMERS

You can see from the above example that if the total burden (wire plus device) is only 2.5 VA, the accuracy of the reading will be

±0.6%. If a higher burden (up to 12.5 VA) is used, the accuracy will be ±1.2%.

For lower-line currents, the accuracy at 10% of the CT rating (120A through a 1200:5 CT) is double the published value. On the

above example, at 2.5 VA burden, the accuracy would be ±1.2%. At a 12.5 VA burden, the accuracy would be ±2.4%.

BURDEN CALCULATION

STEP 1: Determine the burden of the connected device in VA or ohms impedance. This should be on the device data sheet.

STEP 2: Add impedance of the secondary wire run. Measure the length of the wire run between current transformer and the

burden (e.g., meter, relay, transducer, etc.). Refer to Nomogram No. 2 and determine the resistance, in ohms, of the

wires that connect the secondary of the current transformer to the devices. The Nomogram makes allowances for

the return wire. Add this resistance from Nomogram No. 2 to the impedance of the connected burdens. Nomogram

No.1 will help convert ohms to VA.

STEP 3: Make sure the total burden does not exceed the speciﬁed limits for the chosen CT.

If a current transformer (CT) reads low, then the secondary is probably overloaded. Burden is the load which may be imposed

on a transformer secondary by cables and connected devices without causing an error greater than the stated accuracy

classiﬁcation. Lower burden and proper sizing can improve the accuracy of the CT. It is a good idea to check your CT burden if

the CT primary rating is under 200A.

Burden is expressed in ohms impedance or volt-amperes for current transformers. The standard burden limits are deﬁned by

ANSI C57.13.

CURRENT TRANSFORMERS

CTs carry an ANSI burden designation "B" followed by the ohms limit (e.g., B0.1). Below is a typical CT speciﬁcation in our

catalog:

Allowable burden in ohms (0.5 max)

Allowable burden in volt-amperes (12.5 VA max)

Accuracy at burden above (e.g., ±1.2%)

CT primary to secondary ratio

Model # Current ANSI Meter Class at 60 Hz

Ratio B0.1 B0.2 B0.5

2.5 VA 5 VA 12.5 VA

600T-122 1200:5 0.6 1.2 1.2

Line

Devices

(Meter, Relay,

Transducer, etc.)

Wire = Rw

Burden = RW + RD

March 2014

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Delta MCS-1800 Specifications Page 63