Well you SHOULD and need to add R and X values(not Z, that's the resultant impedance), they are not the same thing but none the less can be added, subtracted, divided or multiplied, just not in the conventional way.Rb has been around for years, it's the solid earth of the Tx which is a requirement where T is the first letter of the earthing arrangement.
Apprentice, you worry about Rb if you want a system which utilises MCB's for example, or where its a Private Tx, there are requirement as to the impedance of Rb.
---------- Post Auto-Merged at 14:09 ---------- Previous post was made at 13:32 ----------
Paul you should know better, you teach the 2391, if you draw the fault loop for a TTand don't include Rb you will loose marks.
To the other's, this is fundamental stuff, the fact you aren't aware of Rb is a worry. It's not rocket science, what's the first T stand for, simple stuff, basics.
There also seems to be a lack of knowledge for the math.
I suppose this is due to the dumbing down of our industry.
Most of you work for yourselfs, you should know the theory, you have to design, it's quite worrying.
If, you recall I asked for a reminder, it was late, & cannot recall all aspects of the regs & all the other stuff I do at all times!
With regard to the teaching, yes I do, and, I do teach to put in the source earth rod, but, the students have enough to remember without them remembering the abbreviations!
Yes they should, and yes these should be on the drawing, but, not many people have a photographic memory!
As I don't teach 2391 "every week" when I do have a session coming up I prep for it, by running through the teaching materials and the relevant areas in the regs, I don't have a photographic memory either, and have a lot on my plate.
For all domestic sparks, there is nothing they can do about Rb anyway, so where is little to be concerned with, unless you are looking at a PNB install perhaps, or a single domestic TT/TN from a dedicated transformer, but, again, nothing can be done about it anyway, it is, what it is.
As far das Onoff picking up App for the Rb/RB thing, it is difficult, well I don't think I can on this device add subscripts, it "should" be capital R, with subscript capital A for the earth rod resistance, however, most understand the Ra term.
I did not say that Rb had not been around, I said that as far as I knew it is only currently explicitly mentioned in chapter 44, apart from the definitions.
There may, be more uses, but, In the quick check I did, I could not find them.
If you have a private Tx then,, this is a situation where you are worried about Rb, but would a "run of the mill" electrician be involved in the design, installation, & commissioning of an HV/LV transformer alone? I doubt it tbh.
In a normal situation, would you really have sufficient info to undertake the required calculations, considering that if you are using say a conventional MFT to take the readings, they are measured in different ways, one is a d.c. resistance, the other is actually an impedance measurement?
I never said that you should not add/subtract, resistance and reactance, I stated impedance, and if you read the post, you said approximate, and, I agreed that whilst wrong, it would give you an approximation.
Gonna try to find a way of handling sub/superscripts on this pad,me back later!
Got an hour to kill here.
---------- Post Auto-Merged at 14:53 ---------- Previous post was made at 14:41 ----------
It works!
I could not get the advanced reply to function before, now it does, so:
RA & RB
Woo hoo!
