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Reduced resistance of resistor
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engrguy42:

--- Quote from: floobydust on April 11, 2020, 11:49:20 pm ---
--- Quote from: engrguy42 on April 11, 2020, 11:09:16 pm ---[...] Someone says wirewound power resistors are infallible and cannot go lower value. A well-referenced report says otherwise. As a minimum that should cause someone to be open to the possibility they're wrong.

Basic engineering.

--- End quote ---

Certain safety standards where resistance decreases in value create a hazardous condition, specify using wirewound resistors which are considered "infallible" in them.
The Navy report is too vague to be useful and contradicts the safety standards and decades of knowledge. Perhaps they tested plastic encapsulated wirewound resistors but even then, their core is ceramic and would not deform. I think the report is not an authority.

Stick to your basic engineering, while some of us work with real world designs.

--- End quote ---

Do you have any actual data to support your statements?

bob91343:
A mountain has been created from a molehill.  When measuring resistors of relatively low resistance, errors appear due to dissimilar metals, oxides, lead resistance, and more.

Use a four wire method to elminiate some of these errors.  With my 4 wire ohmmeter I can measure the resistance of a screwdriver shaft.  I made up some four wire measurement leads that I often use to remove the effect of the leads.

Shorting the leads on a 2 wire ohmmeter and then subtracting that amount from your measurement is wrong due to variations in contact resistance as well as thermal effects.  The four wire method is the way to go if you want accurate, consistent results.
vk6zgo:
You aren't dealing with milliohms here.

56 ohms is well within the range of values that are commonly tested with ordinary multimeters.

A short over several (or even more) turns may give a definite resistance drop, but not 5600%.
A far more common fault with WW resistors is to go open circuit.

If there is a reason to use a WW resistor, there is usually a bit of power  involved, so in service, a short circuit would quite rapidly burn out, becoming an open circuit.

My guess would either be a faulty meter, a 0.56 ohm resistor, or a strangely packaged inductor with, in both the latter cases, ambiguous markings.
digsys:

--- Quote from: leftek --- Hello, from one old board a have put out the resistor 56R 9W, that measured with DMM   1ohm.
Can be one resistor have reduced resistance as fault?
--- End quote ---
Looks like you've caused a train wreck here  :)  Can you post a pic of the actual resistor, so we can see exactly what it is ?
floobydust:

--- Quote from: engrguy42 on April 11, 2020, 11:55:00 pm ---[...] Do you have any actual data to support your statements?

--- End quote ---

Safety Standard IEC 60079
intrinsic safety “i”
"type of protection based on the restriction of electrical energy within equipment and of interconnecting wiring exposed to the explosive atmosphere to a level below that which can cause ignition by either sparking or heating effects"

8.5 Current-limiting resistors
Current-limiting resistors shall be one of the following types:
a)   film type;
b)   wire wound type with protection to prevent unwinding of the wire in the event of breakage;
c)   printed resistors as used in hybrid and similar circuits covered by a coating conforming to 6.3.9 or encapsulated in accordance with 6.6.
An infallible current-limiting resistor shall be considered as failing only to an open-circuit condition which shall be considered as one countable fault.
A current-limiting resistor shall be rated in accordance with the requirements of 7.1, to withstand at least  1,5 times the maximum voltage and to dissipate at least 1,5 times the maximum power that can arise in normal operation and under the fault conditions defined in Clause 5. Faults between turns of correctly rated wire wound resistors with coated windings shall not be taken into account. The coating of the winding shall be assumed to comply with the required CTI value in accordance with Table 5 at its manufacturer’s voltage rating."
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