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

Current Sensors.

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

existing system.

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.

• Check that all currents read something sensible — CTs connected;
• Check that all voltages read something sensible — Voltages connected;
• Check the Watt readings. As a guide, 5 Amp on a 400V system is approx. 3.5kW assuming that all three currents are the same & PF = 1.
• If the Watt reading is low (approx. 1/3 of expected), one of the CTs is probably reversed. If the meter is only measuring import power, turning “Auto Rotate” ON in the configuration menu will ensure that the meter corrects for this.
• If the Power Factor is strange, the connection is “Cross Phased” – the measured
  current is associated with a wrong phase.
  
  

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

1.00kV.

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

terms.

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

Cube400 ?

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

operating guide.

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.

MODBUS®

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.