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. | | What is the difference with RMC and IMC? |
Friday, December 23, 2011
IMC v. RMC
Another good question popped up on the first year homework check in page, it is:
Answer: Bottom line is that IMC has a thinner wall but the steel is processed in a way that makes it stronger, so IMC is lighter yet just as strong. For example 1" IMC weighs 119 lbs/100' while RMC weighs 161 lbs/100'. They have the same external dimensions so they thread and bend in the same tools and screw into the same fittings. The thinner wall gives IMC a larger internal diameter which, coupled with an internal coating not always found in RMC, makes it easier to pull wire through. The manufacturers say that bending IMC is just the same as rigid. Your results may vary.
Friday, December 09, 2011
Here are a couple of questions posted to the homework form.
The easy one first:
Question: Question number 14 on Lesson 23, im not quite sure what to answer there.
"What does OSHA require the employer to do concerning a written description of an employment grounding conductor program?"
Answer: read a little more slowly, it is an EQUIPMENT grounding conductor. The answer to your inquiry is to be found on page 141 in the top third of the right hand column.
Two other great questions came in:
Question: what is the differance between ampere rating and Ampacity? And would a Fuse or any type of OCPD work exactly they same if it was DC going through or AC passing through it?
Answer: Check Article 100 of the code book for the definition of ampacity. Ampere ratings are within the context of OCPD's at this point, ampacity ratings have to do with conductors, taking in to account their 'conditions of use'. More on that later.
RMS AC current has the same heating effect as DC. So if the OCPD is responding to HEAT, it will start to trip at the same time for AC or DC. Some OCPD's respond to instantaneous current values through magnetic or electronic means, and so the response is based on the peak of the ac waveform and therefore would trip with a lower value of current for RMS AC versus DC. Another issue this brings up is that AC current drops to zero and changes direction twice a cycle and is therefore 'easier' to interrupt than a like value of DC. Switches and OCPD's for DC need to be specially rated because of this phenomenon.
GREAT QUESTIONS.
The easy one first:
Question: Question number 14 on Lesson 23, im not quite sure what to answer there.
"What does OSHA require the employer to do concerning a written description of an employment grounding conductor program?"
Answer: read a little more slowly, it is an EQUIPMENT grounding conductor. The answer to your inquiry is to be found on page 141 in the top third of the right hand column.
Two other great questions came in:
Question: what is the differance between ampere rating and Ampacity? And would a Fuse or any type of OCPD work exactly they same if it was DC going through or AC passing through it?
Answer: Check Article 100 of the code book for the definition of ampacity. Ampere ratings are within the context of OCPD's at this point, ampacity ratings have to do with conductors, taking in to account their 'conditions of use'. More on that later.
RMS AC current has the same heating effect as DC. So if the OCPD is responding to HEAT, it will start to trip at the same time for AC or DC. Some OCPD's respond to instantaneous current values through magnetic or electronic means, and so the response is based on the peak of the ac waveform and therefore would trip with a lower value of current for RMS AC versus DC. Another issue this brings up is that AC current drops to zero and changes direction twice a cycle and is therefore 'easier' to interrupt than a like value of DC. Switches and OCPD's for DC need to be specially rated because of this phenomenon.
GREAT QUESTIONS.
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