Fault Finding meter Connections - ND - Home - …

1 Fault Finding meter Connections General It is essential that the meter is installed according to the schematics shown in the manuals supplied. If the manual is missing, a copy may be downloaded from the ND metering Solutions website; see The meter requires a separate Auxiliary Supply. This may be connected in parallel with one of the phases or, where wide voltage variations are experienced, from a separate maintained supply. Alternatively, a DC Auxiliary Supply option is available for some ND meters , but must be specified at the time of order. Although ND meters are simple to install, the system into which they are fitted can make commissioning difficult. There may be problems in determining the direction in which the current flows in a cable ( which end corresponds to supply and which end feeds the load) or in fully identifying wiring that has been brought into a control panel.

2 ND meters are not affected by phase rotation (the sequence of the phases). Certain meters have an auto-commissioning feature that aids in identifying the direction of the CTs; this is described in the manual for the meter . Quick Checks No meter display Check that an Auxiliary Supply is connected, and that it is live. Also, that the supply is within the rated limits as specified on the meter label typically 15% of the nominal voltage. Modern ND meters are not fitted with a fuse, protection being provided by other means. The meter keeps trying to re-start The Auxiliary Supply voltage is too low, for example: 110 Vac where the meter requires 230 Installation Errors There are two types of errors that are commonly made when installing current Transformer (CT) operated meters : Reversed current transformers One or more of the CTs is installed or cabled in reverse.

3 This causes the measurement of that phase to be treated as export1 of electricity. If just one CT is reversed, then the meter reading is reduced by 66%. If two or more CTs are reversed, the meter will be showing that electricity is being exported on a 4-quadrant meter a negative kW reading is displayed. Cross connection current & voltage Connections to the phases are crossed over. A 3-phase meter normally contains three individual measuring elements. Each element measures the current & voltage in a phase and calculates power. In a Cross-connected meter , two or more elements are measuring the current in one phase and the voltage from another phase. With two phases crossed and a nominally balanced load, the meter reading is reduced to Zero; or by ~50% if the CTs are also reversed.

4 Northern Design (Electronics) Ltd 13 January 2009 Page 1 of 3 1 Import & Export of Electricity Import is where electricity is supplied by the grid to the installation. Export is where electricity is generated within the installation & supplied to the grid. Fault Finding meter Connections Fault Finding ! ! ! SAFETY NOTE ! ! ! current transformers operate in opposite way to voltage transformers . A current transformer is a source of current . Leaving the secondary OPEN CIRCUIT is the Dangerous condition the opposite of voltage transformers . Unused CT secondaries should always be left short-circuited. The danger can be deduced from Ohm s Law (V = I x R). If a current tries to flow into an infinite load (an open circuit), it will generate an infinite voltage.

5 This warning does not apply to current sensors with an AC voltage output such as those supplied with ND retrofit metering Kits. These may safely be left open circuit. The following Fault Finding sequence should help to detect the more common installation errors. 1 Isolate all inputs to the meter . 2 Check that wiring to the meter is in accordance with the instructions in the manual. Check all Connections . Check the wiring between the CTs and the meter , looking for a possible reversed CT. If a CT has been reverse installed, it may be simpler to reverse the secondary Connections to the meter rather than re-installing the Check that the CT connected to CT1 is measuring the current in the phase connected to L1; similarly for CT2 / L2 and CT3 / L3.

6 If such an error is found, it is usually simpler to change over the voltage Connections . Recheck all Connections . 3 Energise the inputs to the meter , both measurement and auxiliary supply. 4 Check that the meter powers up. If the meter keeps trying to re-start then the auxiliary supply voltage may be too low, for example: 110 Vac where the meter requires 230 Vac. If the meter does not power-up, check the auxiliary supply. 5 Ensure that there is a load for the meter to measure. Ideally this should be at least 5% of the rating of the CTs, and certainly no lower than 2%. At lower currents, it can be difficult to identify errors. 6 For each phase, note the voltage, current , Power Factor (PF) and Power. Confirm that the currents and voltages are within the range expected.

7 Check the PF. This would normally be better than However, if the load is purely lighting, the PF may be lower. For compact fluorescents, the PF can be as low as A negative PF reading would indicate a capacitive load. If such a reading is obtained with an inductive load, then there is almost certainly a wiring error. Check the power reading (in kW), comparing the measured power to the measured voltage and current readings. Note that power should equal voltage x current x PF. For example; in a 400 volt system with a PF between and 1 the power for each phase should be between 17 kW and 23 kW per 100 Amp. If the load is mainly lighting, particularly compact fluorescents, the PF could be as low as and thus the power could be as low as 11 kW per 100 Amp.

8 Northern Design (Electronics) Ltd 13 January 2009 Page 2 of 3 2 CTs are symmetrical. Reversing the input has the same effect as reversing the output. Extra care must however be taken if the CTs are connected to two or more devices or where the S2 Connections are commoned (Star connected CTs) Fault Finding meter Connections If one of the phases seems to have a correct reading, while for the other two the power & PF readings seem wrong, then the latter two phases are most probably cross connected. Recheck the wiring, and if cross connection is confirmed switch over the two voltage Connections at the meter 7 If all three individual phase readings seem correct, check that the total 3-phase power is as expected.

9 If not, repeat the steps to particularly checking the polarity of the Power reading. In most circumstances (where the PF is greater than on all phases), the combination of Connections that gives the highest Power is normally correct. 8 The meter should now be reading correctly. Northern Design (Electronics) Ltd 13 January 2009 Page 3 of 3