ND CT Cable Losses
When specifying Current Transformers, the burden (or load) imposed by the cable must
be taken into account. The terminations of the cable at the CT and at the Meter are here considered to be secure and of negligible resistance (less than 0.01, the equivalent of 0.5VA in total at 5 Amp).
If the total burden exceeds the rated value of the CT, accuracy will not be maintained
no matter what current is flowing. Thus, for a CT with a 5 Amp secondary rated at
5VA, the total burden must be less than 0.2oHm.
The chart below lists the resistances of various cable types and lengths and the
equivalent burdens at a current of 5 Amp. At 1 Amp, the burden is a factor of 25 less.
What type of Current Transformers should I use with ND’s Meters ?
A. This depends on the meter you have.
If the Meter is a Multicube Modular, Rail350V, Cube350V, Cube400V, Cube650V or
Cube950V (in essence the model type ends in ‘V’), the meter is designed to use Current
Sensors with a 333.3mV output. In many cases, it will have been supplied with such
Otherwise, the Meter is designed to use standard Current Transformers — normally with a
5 Amp (optionally 1 Amp) Current Output.
How do I select current transformers for use with ND’s Meters ?
A. If your Meter is designed to use 333.3mV output Current Sensors, if no Current Sensors are
provided with your Meter, contact ND Metering or your local partner for these devices. ND
carefully selects suitable Current Sensors and the Meter’s firmware ensures maximum
accuracy with these devices. Although other 333.3mV sensors can be used, such devices
may affect overall accuracy.
If your meter is designed to use 5 Amp CTs, see below.
How do I select current transformers for use with ND’s Meters ?
A. If your meter is designed to use 5 Amp CTs, quality measurement CTs should be used.
These should be Class 1.0 (BS EN / IEC) 60044-1 or BS EN IEC / IEC 61869-2) or better in
order to provide accurate readings over a wide range of loads. The size and format of the
CTs should be selected to suit commissioning. The hole in the center of the CT should be
big enough to fit over the cable and the outside size should fit in the space available.
How do I rate my CT primary ?
A. The CT primary should be selected to match the maximum Amps expected in the system
being measured. Optimum accuracy is achieved if the meter operates with inputs near
100% of rated input. A common mistake is to select the CTs to match switch ratings, which
may be much higher than the actual current flowing.
Both CTs and Meters measure accurately up to 120% of the nominal rating and can
withstand higher short-term overloads.
What does the P rating on my current transformer mean ?
A. 'P' rated CTs are designed for input to “Protection” relays and are linear up to x10 or x20 of
the nominal rating. Although such CTs may also be rated for metering accuracy, sustained
overload currents could damage the meters. ND’s 5 Amp input Meters are rated to
withstand a x20 overload for 0.5 sec., however a longer overload could damage the Meter.
If such overloads are expected, either special 5A:5A CTs should be fitted to protect the
Meter or separate metering CTs should be fitted.
What VA rating is required for my current transformers ?
A. ND Meters take very little power from the CT secondaries. Most of the load on the CT will
come from cabling and terminations. For short cable runs with 2.5mm2 cable, a 2.5VA CT is
sufficient for a 4m cable run; with 4mm2 cable, a cable run may be up to 8m. See ND CT Cable
Losses for more information.
Is it possible to wire my Meter without disconnecting the load ?
A. For safety, the load should be SWITCHED OFF before fitting retro-fit current sensors.
The best option is to use ND ....V Meters which are optimised for use with 333.3mV split
Current Sensors. Standard 5 Amp meters could be used with split-core CTs but these are
much larger and need to solidly mounted.
How close to my meter must my CTs be ?
A. If 333.3mV Current Sensors are being used with ....V Meters, the cabling can be extended
up to 100m. This only applies to cables for the CT output. The cable must be 250V rated
& screening is recommended. Suitable cables include Belden 9841, Alpha 6412 and
equivalents. If 5 Amp (or 1 Amp) CTs are being used, VA loading on the CT secondary needs to
be kept to a minimum. A 5m run of 2.5mm2 cable imposes load of 2VA on the CT at 5 Amp;
with 4mm2 cable, this reduces to 1.2VA. If longer cable runs are required 1A rated meters
need to be specified. At 1A, the load imposed on the CT is reduced by a factor of 25, namely I2
Can I power my meter from the measurement signals ?
A. Yes. The auxiliary mains input to the meter can be fed from any suitably rated supply. It is
common practice to wire one of the measurement phases and neutral across to the auxiliary
supply inputs at the meter. Keeping the auxiliary input separate allows a bank of meters to
be powered evenly amongst the available input phases. It may sometimes be necessary,
where the input voltages vary wildly, to connect the meter supply to a stable voltage, an
option not available on many competitive products.
Can I connect my ND Meter using existing CTs/PTs ?
Applies to all ND Meters with 5 Amp or 1 Amp inputs.
A. Yes. If existing CTs and/or PTs are of suitable types and suitably rated the Meter may use
these for its inputs. A single Meter’s measurement inputs will add very little load to the
NOTE: Wire currents in series and voltages in parallel.
What happens if I open circuit my CT secondary’s while a primary current
is present ?
A. BANG is a possibility on a high current circuit !.
The secondary of the CT will attempt to drive 5A into the load provided. If this load is open
circuit (infinite ohms) then V=IR (ohms law) states that the output voltage will attempt to
reach infinite volts. Only the physical properties of the CT will limit this. Dangerous flash
over voltages can be achieved under these conditions.
NOTE: This does NOT apply to the special current sensors supplied with PowerRail 350V
and Cube 350V Retro-fit Metering kits.
Do I need separate fuses for my voltage measurement inputs ?
A. Fused voltage measurement inputs are recommended practice for all instrumentation.
These protect the main system under instrumentation fault conditions. Removable fuses
also provide a practical means of isolating the meter input voltages during commissioning.
The fuses used may be used for other instrumentation.
What fuse rating should I use ?
A. Normally, the smallest current rating for Breaking Capacity.
The consumption of the meter is negligible. 3-4 VA for the auxiliary and less than 1mA for
each measurement circuit.
Can my ND Meter measure single Phase ?
A. Yes, but use the L1 channel. All ND Meters have based three independent measurement
elements. Displayed parameters associated with unconnected inputs will read zero on the
LCD. If using ....V Meters, it is a good practice to short the other two current inputs.
What is the maximum load I can measure with my ND Meter ?
A. The maximum load is defined by the external CT and PT primary ratings. The meter is
programmed to suit these. For the ....V Meters, the maximum CT primary that can be
programmed is 25,000A and the maximum PT ratio is 1000.
How do I check if my ND Meter is connected correctly ?
A. All modern ND Meters models display per phase Amps, Volts and kW. These readings are
usually sufficient to determined correct connection.
NOTE: ND has developed a small programme to identify to identify which
phases have been incorrectly connected.
How is display of kWh etc scaled on ND Meters ?
A. Energy readings are automatically scaled to provide a sensible accumulating rate for each
register at full scale. Automatic decimal points and Wh, kWh, MWH legends are used to
achieve this. The DP and legend will not change unless the CT and/or PT programmed
setting are changed. TheDP and legend are set to provide a rate of accumulation with full
scale inputs in the range of 1 count every 1.5 Seconds to 15 seconds.
How is display of Volts, Amps, kW etc scaled on ND Meters ?
A. Single phase Instantaneous readings are scaled to provide an optimum resolution of 4
digits on the LCD Display. Over range (120% of nominal) is taken into account with the CT and
PT programmed settings to automatically set the decimal point and legend. For example: a PT
setting of 33,000V will have an over range vale of 33,000x1.2=39,600V. This can be displayed
in 4 digits as 39.60kV. If the input voltage is wound down to 1000V this meter will display
What does Class 1.0 mean ?
A. Class 1.0 is a term used in metering standards to specify the accuracy of the meter under
specified operating conditions and over a standardized operating range. For example a
meter conforming to EN / IEC 62053-21, Class 1.0 means measurement to within 1.0% of
reading over a current input range 5% to 120% of rated input at PF=1.0. This standard also
specifies limits of error for PF=0.5 and PF=0.8. A full understanding of each standard is
necessary to fully appreciate the precise meaning of the classification. Some companies
claim to conform to standards over narrower operating ranges, which is a contradiction in
NOTE: EN / IEC 61036 was obsoleted in 2003
What is the difference between error % reading and % error full scale ?
A. % error of reading defines how inaccurate a value is with respect to the true expected
reading. % error of FS reading defines the inaccuracy as a proportion of the nominal full
scale value. % error of FS permits greater errors as the load decreases. At very low loads
% error of rdg rises very quickly.
Example a meter rated at 500.0A is fed with 10.000A; It reads 10.01A on the LCD.
The % error of reading is calculated as (10.01-10.000)/10.000 = 0.1% rdg
The % error of full scale is calculated as (10.01-10.000)/500.0 = 0.002% FS
Are ND Meters suitable for measurement on non-linear loads (e.g.
electronic motor speed controls) ?
A. Yes. ND Meters use a very fast & continuous sampling system. By taking a large number
of samples very quickly it is possible to measure distorted waveforms. Continuous
measurement allows jittering or rapidly fluctuating loads to be measured accurately. Burst
fire systems may also be detected but the LCD display of the unit will reflect the ON/OFF
nature of the load, which may be unacceptable to the customer. Voltage/Current demand
may be used to reduce this effect.
How often do ND’s Meters measure ?
A. ND’s Meters samples the voltage and current signals continuously, never pausing,
calculating raw readings on the fly. Once a second the results of the sampling process are
converted to measured parameters using calibration constants and user programmed
settings to provide the final scaled readings. The LCD display and Options Unit values are
updated with these results once each second.
NOTE: Some meters such as the 950 series have a Fast mode where readings are
updated every 100ms.
Should I reset my meter readings after programming ?
A. Measurements continue even during programming. If the CT and or PT settings are
changed during programming then historic energy and Peak readings, calculated with the
old settings, may now be nonsense. It is advisable to reset energies and peaks after
changing CT and PT settings.
What is the ambient operating temperature for ND’s Meters ?
A. ND’s Meters may be operated in an ambient temperature of –10 deg C to 65 Deg C.
What is CT Auto-Rotation or auto correction ?
A. The most common of all wiring errors when installing meters is to install a toroidal CT the
wrong way round on the conducting cable. This effectively reverses the current signal
causing negative (export) readings for kW. ND’s Meters automatically detects this type of
error and uses software to correct for the subsequent errors in measurement. Auto-
Rotation can be turned OFF if the meter is measuring both Import & Export energies.
What is special about a “Wide Viewing Angle LCD” ?
A. Low cost LCDs provide good contrast readings only if they are viewed from directly in front
(“head on”). With this type of display a panel-mounted unit must be mounted at eye level, a
difficult concept with users of varying height. Another problem with this type of display is
viewing from the side as happens when taking readings from many instruments in a large
panel from a fixed standpoint. A higher quality wide viewing angle LCD provides high
contrast displays from all angles (except from behind the panel!).
What happens to my energy readings if I lose auxiliary power ?
A. Energy readings are stored in a non-volatile memory in the event of auxiliary power failure.
The unit will store the values securely for a minimum of 10 years until the power is restored.
The 10-year period starts after each power failure. Instrument programmed settings and
calibration are stored in the same memory.
Can I reset my energy readings ?
A. Yes. All energy readings may be simultaneously reset using the front panel keys as
described in the manual.
NOTE: Does not apply to MID approved Meters.
Can I disable the energy reset feature ?
A. Plug-in configuration links may be fitted to disable the energy reset facility on a meter. This
may be used as an anti-tamper device. This change must only be made by authorized
MultiCube service engineers and is not documented in the standard operating guide.
Can I disable the energy reset feature on the Cube300, Cube350 or
A. A plug-in configuration link situated on the top of the Meter near the terminals may be
inserted to disable the energy reset facility on a meter. This may be used as an anti-tamper
device. This change must only be made by authorized service engineers and is not
documented in the standard operating guide.
Can I disable the energy reset feature on the Rail 350 ?
A. A plug-in configuration link situated in the Meter. It may be inserted to disable the energy
reset facility on a meter. This may be used as an anti-tamper device. This change must
only be made by authorized service engineers and is not documented in the standard
What is the function of the pulse outputs on an ND Meter ?
A. These provide isolated signals, which correspond to the accumulation of kWh and kvarh
registers (if fitted). A single pulse is provided for each count in the associated register. The
user may set the output to pulse after every 1, 10, or 100 counts to suit external systems.
The pulse output can be connected to a BMS system or remote counters.
What is the difference between RS485 and RS422 Communications ?
A. RS422 uses 4 signal wires, 2 for transmit and 2 for receive. It is similar to RS232 but uses
different voltage/current levels to allow communication over much longer distances.
RS485 uses only 2 signal wires. The master unit and each MultiCube must have in built
intelligence to “Turn Round” its driver outputs as it needs to Transmit or Receive.
is a software protocol that knows nothing about hardware devices. MODBUS
protocol ensures no device needs to transmit and receive at the same time. The master
unit (PC) sends out a request addressed to a single unit on the bus. The addressed unit
replies after a short delay giving the master time to enter receive mode.
Either RS422 or 485 will work with MODBUS. RS485 is normally selected, but may
occasionally cause problems where the PC cannot turn round quickly enough after sending
a command. This can happen when working with an older (and slower) PC under
multitasking operating environments such as Microsoft Windows.
RS422 is no longer available on current ND Meters.