Current Transformer Developments

Traditional current transformers provide a current output to the meter. The CT is rated for both its primary current and its secondary current, the primary being the level of current flowing in the circuit it is to measure, the secondary being the current flowing in the CT’s output circuit (the one connected to the meter)

The secondary current rating is most commonly 5A, and all current input AEM meters are configured for this value. The primary rating must be selected to match the rating of the circuit to which the CT is being fitted, so a 400A supply requires a CT with a primary rating of 400A or higher. Generally the CT primary value is matched as closely as possible to the supply rating to ensure the maximum accuracy.

However, the provision of a 5A secondary output requires the CT to be significantly larger and heavier than is really necessary for modern digital meters. The 5A rating is essentially a throwback to the era of electromechanical meters, but it is nevertheless an 'industry standard' that cannot be jettisoned overnight. An alternative 21st century approach is the ‘voltage output’ CT.

In simple terms this design allows smaller, lighter, CTs to be used, which offer the additional advantage of virtually unrestricted cable runs for their secondary connections. These CTs can only be used with meters specifically designed to work with them, but where new meters are being installed they provide some significant advantages.

AEM33s are available in standard and 'V' versions, the latter being suitable for use with voltage output CTs. However the following rules should be noted:

• Always use the correct CT type with the correct meter type. Failure to do so will damage either the meter, the CT or both!
• Voltage output CT's are used with 'V' type meters only
• 'V' type meters cannot be used to replace existing meters unless the existing CTs are also replaced