Download Basic Electricity and Heat Transfer

Basic Electricity
Basic Electrical Terms
• Voltage: the driving force, potential
– symbol is V, units are Volts
• Resistance: resistance to flow
– symbol is R, units are Ohms
• Current: a measure of flow
– symbol is I, units are Amps
• Power: rate energy is produced or used
– symbol is P, units are Watts
2
Voltage
•Water makes a great
analogy for
electrical systems.
•Higher voltage is
just like running a
pump at higher
pressure.
3
Current (amps)
• The fish are
swimming in
current (water).
• In a piece of
copper wire the
electron moves
from orbital ring to
orbital ring of
adjacent atoms.
4
Resistance (ohms)
5
Resistance differences
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The analogy
V
I
R
7
Ohm’s Law relates these terms
Ohm’s Law:
E=I•R
I=E÷R
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Distribution system components
Utility
Transmits at high voltage
to reduce line losses
480, 600,
1K, 10K Vac
Reduces voltage
to useable levels
Distributes current and
protects circuits
120, 208,
277 Vac
Heating cable
Transformer
Subpanel with
circuit breakers
Uses electricity to
generate heat
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Standard Voltages
•Commercial and industrial facilities
– 120, 208, 277, 480, 600 Volts, 3 phase
•Residential buildings
– 120 and 240 Volts
•We differentiate our products by voltage
– 120 Volt vs. 240 Volt
– 120 Volt products are designed for 100 – 130
Volts
240 Volt products are designed for 200 – 277
Volts
10
Standard circuit breakers
• Provide over-current protection
for short circuits only
– Trips if current flowing
exceeds breaker rating
• Typical ratings for heat-tracing
applications:
– 15, 20, 30, 40 and 50 amp
• Manufacturers:
– Square D, GE, Cutler-Hammer
11
Ground fault circuit breakers
• In addition to over-current protection, these breakers
trip if a small fault current flows to ground
- 5 mA level for personnel protection
(GFCI)
- 30 mA level for equipment protection
(GFEPD)
• Required for all heating cable circuits by NEC, CEC and
by Tyco Thermal Controls
• Tyco Thermal Controls recommends 30 mA level
equipment protection to avoid nuisance tripping
• Selection Guide on Web site shows breakers available
for different voltages; available in Tyco Thermal
Controls price list
12
Ground fault breaker operation
Circuit Breaker
Heating Cable
Line A
Line B
Fault
Ground
Sensor
If currents in Line A and Line B are not equal, some current is
going to ground through a fault. The ground fault circuit
detects this imbalance and trips the circuit breaker.
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Electrical Issues in Heat-Tracing
•Maximum circuit lengths
–
–
–
–
Self-regulating cables draw more current at start-up
temperature than at steady-state temperature
Thus, maximum circuit lengths are based on start-up current,
not steady-state current
Tyco Thermal Controls calculates maximum circuit lengths to
ensure current draw does not exceed breaker rating
Always use the design guide or TraceCalc for circuit lengths
•Transformer sizing
–
Safe transformer sizing is based current draw at start-up
temperatures rather than at steady-state
– A conservative solution is:
transformer load = breaker rating x 0.8 x voltage
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