metering knowledge
Open CT circuit
Open circuit condition in a current transformer (CT) can result in dangerous over voltage condition at the secondary terminals of the CT. An open circuit CT especially of high ratio and carrying high currents can produce secondary open circuit voltage in the range of few kilo volts. This voltage is usually sufficient to sustain steady state arcing between CT shorting blocks and is a potential fire risk.
B.I.L.
Basic Impulse Insulation Level; A specific insulation level expressed in KV of the crest value of a standard lightning impulse.
Basic Metering Formula
10,000 x Kr = Kh x Rr x Rs x CTR x VTR
Class (Meter)
The maximum load range in amperes for a watt-hour meter. The meter will remain accurate and will withstand class amps with no ill effects
Kh
Disk Constant. Defined as the number of watt-hours required to cause the disk to make one complete revolution. Typical disk constants are 1.8, 7.2, and 14.4
You get a job for a c/o, on a 3ph 3w 480v svc 12s meter, the question is can you c/o the svc at the meter base? ( by booted the meter, * no d/c or safety socket)
NO, because on a 5 jaw meter you can single phase the svc and damage customer pump. ( can c/o at pole )
N.S.V.
NOMINAL SYSTEM VOLTAGE (kV 0.6)
CELSIUS
5/9 x( F Degrees + 32 Degrees) example: 30 degrees Celsius = 86 Fahrenheit [ 30x1.8+32 = 86 ]
Accuracy
The extent to which a given measurement agrees with the defined value
Rs
Shaft Reduction. Defined as the gear reduction between the disk shaft and the first register gear. The disk shaft normally has a worm gear cut on the shaft. Typical shaft reductions include 50 and 100.
EXAMPLES OF CT CIRCUIT FAILURES
CT with Shorted Turns
Wrong Selection of Dual Ratio CT
Magnetized CT
Burden Failure in Secondary Circuit
Wiring Error
Burden
The load, usually expressed in ohms (CT), or volt-amperes (VT) at a specific power factor, placed on instrument transformer secondary by the associated meter coils, leads, and other connected devices.
Time Load check
Watts Load = Khx3600x#revs / time(seconds)
Demand
The average value of power or related quantity over a specified interval of time.
Rr
Register Ratio. Defined as the number of revolutions of the first gear of the register required to cause the first dial pointer to make one complete revolution. The first gear of the register is defined as the gear that meshes with the worm gear on the disk shaft. Typical register ratios are 13 8/9, 55 5/9, and 1111/9.
get a job to meet an electrician, and he wants to do duo runs. the existing meter base is a 305a EUSERC. can they? the electrician says they do this all the time in Portland
NO. electrician would have to go to a junction box or upgrade to ct rated with a 3ph svc.
Ratio
A current transformer (henceforth called a CT) is designed to produce a secondary current, Is, that is directly proportional to its primary current, Ip, which is also called the load current. In metering this proportionality is called the ratio of the CT
power factor
ratio of true to apparent power (PF=kW/kVA)
Eddy Currents
Currents resulting from voltages which are introduced in a conducting material by a variation of magnetic flux through the material
definition of basic metering formula
By definition one revolution of the right hand dial on the register is equal to 10 kilowatt-hours or 10,000 watt-hours
What are the 4 Codes and Standards in Meter Operations
ESR,EUSERC,NEC,AND OREGON PUC
Rating Factor
Some CT’s are certified to be accurate for primary current values greater than 100 percent of the rated ratio, called over-range. This over-range is designated by a rating factor representing the factor by which the upper limit of the primary current can be increased and still remain within certified accuracy. Hence a 200 to 5 rated ratio CT with a rating factor of 2 will remain within its accuracy certification up to 400 amps (200 amps times 2). The rating factor of the CT is stamped on the nameplate of the CT
SINGLE PHASE- AMPERES WHEN KVA IS KNOWN
KVAx1000 / E
EMF
Electromotive Force. The force which tends to produce an electric current in a circuit. The common unit of EMF is the volt.
CTR
Current Transformer Ratio