relay burden

You have to refer to the relay manufacturer's catalogues/data sheets only. The burdens will differ for different manufacturers. One point to note is that in Electro-mechanical Relays, the burden will be the highest at the lowest tap and it must be considered. In numerical relays the burden is constant irrespective of the setting.

Dear Guest,

As we know, burden in current transformer terminology is the load connected to the secondary terminals and is expressed either as VA (volt-ampere) and power factor at a specified value of current, or as total ohms impedance with the effective resistance and reactive components. The term burden is used to diffrentiate the current transformer load from the primary circuit load.

In the calculation CT accuracy, the burden included in terminal ct such as wire, ct, relay, watt meter, power factor meter and etc. For analog meter and electromechanical relay, the burden is stated on each equipment, but for numerical relay it is like only one value for all.

The formula for relay burden (Zr) = VA/(I square) ohms for CT or = (V square/VA ohms for VTs. While wire and CT, burden Zw = equal to r + jx for wire, Zct = r + jx for CT. In calculation x is neglected some time.

Total burden for normal current is Ztotal = Zr + Zw +Zct + Zo

Zo is other burden if the circuit contain other equipment such as watt meter, power factor meter, transducer and etc for electromechanical equipment.

For numerical relay Zo is omitted.

Please correct me if it is not correct.

Regards.

Select your relay burden in line with your CT burden,look at the datas or specs on the equipment body,they should agree electrically.

Check your specifications or other related information on this subject.

Patrick Whowha

You have refer it on your relay manufacturer's data. The value could be on units : VA or ohms.. You only have yo simply add it. but very careful in electromechanical relay if it is per phase installation. You have to consider the fault type and then its equivalent circuit. As for numerical relays, you simply have to add it. It should be on same units. But better if it is on ohms. But there is still an easy way of this. Use an instrument that can measure relay or ct burden... By the way, don't forgot the impedance of your control wire.. you have to include it. In our company, for CT burden evaluation, we used that instrument to measure the total CT burden after the installation of the relays are done. This is just a check up if the CT have carry this burden..