For safe fusing, choosing a flameproof resistor is necessary, but it is not always sufficient. Flameproof status simply means that the resistor coating will not support sustained combustion. However, a flameproof resistor can cause smoking and even fire under moderate overload conditions, as very high body temperatures can be reached prior to fusing. This can cause charring of the PCB or ignition of adjacent components. The two defenses against this are defined fusing characteristics and PCB standoff leadforms.
The pulse performance of typical products from TT electronics is shown in Figure 2, below. Often these peak voltages may be raised by up to 100 percent by special winding design, enabling cost savings through the use of a smaller body size. Note that at ohmic values above about 1K0, it may be possible to use metal oxide technology as a lower cost alternative to wirewound.
When calculating the peak voltage across the line input resistor, it is necessary to subtract the clamping voltage of the MOV, typically 700V to 1,000V, from the peak voltage applied to the circuit.
Figure 2: The pulse performance of resistors from TT electronics.
Voltage measurement
The voltage measurement input is derived by resistive division of the line voltage. This entails direct connection to the line input and therefore exposure to the same high-voltage pulses as the line input resistor. However, as the divider feeds a high-impedance input, much higher ohmic values (typically 470K to 1M0) may be used, so pulse energy is correspondingly reduced.
This is often achieved by means of a series chain of between four and eight MELF resistors, which provide the required stability, and in series combination, the required rating and pulse capacity.
Excellent article, very thorough and detailed. Few people realize how difficult it can be to specify a simple resistor. It's also quite common to use two resistors in series to satisfy redundant failure analysis.
Resistor consideration is related to instantaneous or repeated peak MOV power dissipation, too. Few engineers realize that a lightning stroke does not consist of just one pulse, but five or more together. I proved this years ago with a camcorder, and viewing a lightning strike frame by frame I saw separate bolts of lightning in every other video frame.Our eyes perceive a lightning strike as flickering, but in reality each strike is a series of discharges until the cloud voltage drops low enough to stop altogether.
Consider each meter is also sealed environment with a glass cover, much like a greenhouse but without ventilation. Now consider a typical meter sitting on a outside, south-facing wall on a home in the deep south. Internal temperatures can easily rise up near the upper operating limits for most electronic components.
Both MOV and resistor power ratings must be greatly de-rated to insure long term reliablility of both. De-rating is also required for other electronic components. Proper testing of components requires both good equipment and use of an environmental chamber during testing. This type of testing reveals flaws and design issues with components not previously apparent or even considered as a problem.
With record-breaking temperatures around the country, it's hard to find a more punishing environment for electronic components. Perhaps a oil well probe lowered down to the botton of an oil to make temperature and resistance measurements might be a worse environment, but that's just a temporary high temperature environment, while a electric meter is on-going day after day A wall ona building is even worse than the passenger compartment in most vehicles. At least in the case of vehicles, electronics can be located under car seats or behind the dashboard, away from direct sun exposure.
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