djr3203
Electrical
- Aug 3, 2011
- 57
Today I was looking at the as-built drawings for a physics lab that is adjacent to a lab I will be renovating. I noticed on the electrical plans that the engineer provided isolated/insulated grounds to all the receptacles and also called for the branch circuits to be twisted with a twist rate of 4" per twist cycle.
Now my fundamentals of electric theory and communications may be a bit rusty, but every time I have read about twisted pair it had to do about common-mode noise reduction with differential mode signaling. The reason for the twisting, as my understanding goes, is twisting having both wires be equally affected by the incoming interference. Since the signal is differential, when the receiver subtracts the two signals the noise gets cancelled out since it (hopefully) has the same magnitude and phase on both wires.
Lately I have heard some people thinking that twisting of the feeders will help reduce emitted RF interference from AC feeders/branch circuits. When it comes to 3 phase, I think this will only help marginally since each phase is 120degrees out of phase and should cancel. I guess twisting could help get the cables closer together which could help keeping the emitted EM at 120degrees out of phase with each other helping them to cancel out?
In my opinion, running the feeders/branch circuits in grounded steel conduit is the best way to reduce EMI.
I guess my questions are:
1. Will twisting the wires in a branch circuit actually cause more trouble than good? I mean twisting the wires would increase the inductance which would increase any mutual inductive coupling between adjacent twisted branch circuits run in the same conduit?
2. Would twisting the wires increase the inductance and possibly cause higher voltage transients from switching or loads coming on/off on the circuit?
3. Would twisting provide any benefit to decreasing emitted RF interference from a single phase branch circuit?
4. Do 2 wire parallel branch circuits emit RF interference?
As always, thanks for any info you can shed on the subject.
DJR
Now my fundamentals of electric theory and communications may be a bit rusty, but every time I have read about twisted pair it had to do about common-mode noise reduction with differential mode signaling. The reason for the twisting, as my understanding goes, is twisting having both wires be equally affected by the incoming interference. Since the signal is differential, when the receiver subtracts the two signals the noise gets cancelled out since it (hopefully) has the same magnitude and phase on both wires.
Lately I have heard some people thinking that twisting of the feeders will help reduce emitted RF interference from AC feeders/branch circuits. When it comes to 3 phase, I think this will only help marginally since each phase is 120degrees out of phase and should cancel. I guess twisting could help get the cables closer together which could help keeping the emitted EM at 120degrees out of phase with each other helping them to cancel out?
In my opinion, running the feeders/branch circuits in grounded steel conduit is the best way to reduce EMI.
I guess my questions are:
1. Will twisting the wires in a branch circuit actually cause more trouble than good? I mean twisting the wires would increase the inductance which would increase any mutual inductive coupling between adjacent twisted branch circuits run in the same conduit?
2. Would twisting the wires increase the inductance and possibly cause higher voltage transients from switching or loads coming on/off on the circuit?
3. Would twisting provide any benefit to decreasing emitted RF interference from a single phase branch circuit?
4. Do 2 wire parallel branch circuits emit RF interference?
As always, thanks for any info you can shed on the subject.
DJR