A technician checks the resistance on a delta wired 3 phase 240 V 15 kW electric duct heater. If one element is open, what resistance would he read?

When assessing the resistance readings across a delta wired electric heater, it's important to understand how the configuration of the elements affects the measurements, particularly when one element is open.

In a delta configuration, each heater element connects between two of the three phases (L1, L2, L3), forming a closed loop with the potential to measure resistance. When one element becomes open, it interrupts the circuit for that particular phase pair, leading to distinct resistance readings:

1. The remaining two elements are connected to the resistor pairs that can be measured. Since the heater is rated at 15 kW on a 240 V system, you can calculate the resistance per element.

2. Each element has a resistance calculated using the formula: Resistance (R) = Voltage² / Power. For 15 kW and 240 V, the total current can be derived and the individual phase (or element) resistances can be determined.

3. If one of the three elements is open, it is expected that you will measure the resistance values between the two remaining phases where the elements are still intact. The combination of these two elements produces a resistive reading, but the open element results in infinity resistance between the phase pair that includes the open element.

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