Why are power supply and test equipment leads not in a twisted pair arrangement?

[SharpEars]

Wouldn't that make sense to reject some (common mode) noise at the cost of a tiny bit of inductance? I have always wondered this, since 90%+ of the time both the + and - ends of the connectors (e.g., assuming DC) are going to points in close proximity in a circuit.

Note: I am excluding high bandwidth oscilloscopes from discussion, which are obviously affected by event that slight bit of inductance...

[Hugoneus]

Wouldn't that make sense to reject some (common mode) noise at the cost of a tiny bit of inductance? I have always wondered this, since 90%+ of the time both the + and - ends of the connectors (e.g., assuming DC) are going to points in close proximity in a circuit.

Note: I am excluding high bandwidth oscilloscopes from discussion, which are obviously affected by event that slight bit of inductance...

In the case where very sensitive measurements are to be done, you would use a triax cable anyway, with guard voltages. As you have pointed out, normal cables are not suitable for sensitive measurements.

[Lightages]

I have seen twisted pairs of wiring for internal construction, so it is not uncommon. It is an extra cost and if the benefit is not that great many companies will just choose the cheaper route. As far as test leads go I don't think a twisted pair of multimeter probes would be very convenient

[Richard Crowley]

In the case of power supply leads, the miniscule benefit you might gain from twisting the leads (in those cases where it is even possible) is greatly swamped out by the large capacitance for bulk filtering and distributed circuit bypass capacitors.

In the case of test leads, if you are measuring something that might be sensitive to ambient interference, then the proper method would be to use shielded wiring.

In the pro audio world, some people are using "super-twisted" unshielded pair (i.e. Cat5 network cable) for BALANCED audio connections, both line-level, and even mic-level.  Of course, without the shield, there is no ground connector to provide a return for phantom power (as used for condenser mics, etc.)

[AG6QR]

For a multimeter, very often I'll use a different style of lead on the positive versus negative terminal.  I'll often clip the negative lead to a ground point, and use a sharp probe on the positive lead.  That way, I often only have to handle one lead.  If I'm using a bench power supply to power the circuit under test, I might use a banana plug lead to connect the common terminal of the meter to the negative of the power supply instead of using a clip lead.

Even when I'm using the same style probes on both positive and negative leads of a meter, there's no telling how far apart I need to place the probes.  So usually, I don't want them twisted.

For a power supply, the supply itself is normally low impedance and at least moderately high voltage, so noise pickup usually isn't a serious concern.

There can be times when noise immunity is really important, but these are the exceptions, not the rule.  It's not hard to switch to some variety of shielded cabling when that's called for, while keeping the convenience of separate leads for those cases where shielding isn't justified.

[SharpEars]

There can be times when noise immunity is really important, but these are the exceptions, not the rule.  It's not hard to switch to some variety of shielded cabling when that's called for, while keeping the convenience of separate leads for those cases where shielding isn't justified.

In the case where very sensitive measurements are to be done, you would use a triax cable anyway, with guard voltages. As you have pointed out, normal cables are not suitable for sensitive measurements.

The problem with *ax type cable is current limitations of the very thin center conductor (and thin shielding, too). The impedance of the cable may also be detrimental.

For example, I can twist flexible stranded 8 AWG or even 4 AWG wiring pretty tightly and still allow for their respective high current carrying capacities. Granted, capacitance and inductance would go up, but resistance would still remain quite low. I suppose in the case of a power supply, the low resistance of the supply combined with the low resistance of thick (low AWG) leads would eat away at most of the common mode noise.

Using coax or triax for that kind of current seems impossible or prohibitively expensive. For example, consider the case of transferring 3.3 V at a very high current (say 25 A) over coax/triax.

[Richard Crowley]

Nobody is suggesting using coax cable for power wiring.

You asked TWO different questions (one about power supply wiring, and one about test leads) and you seem to be confusing them together.
Your two use-cases have NOTHING to do with each other and couldn't be more different.

[SharpEars]

Nobody is suggesting using coax cable for power wiring.

You asked TWO different questions (one about power supply wiring, and one about test leads) and you seem to be confusing them together.
Your two use-cases have NOTHING to do with each other and couldn't be more different.

Actually, they have a lot in common within the context of my question - both apply to and affect a circuit or device under test:

Power Supply Leads
Resistance losses added due to the gauge and length of wire (coax vs twisted low AWG)
Power handling ability due to the gauge of wire
Noise pick-up due to increases in (power supply) resistance (which is not zero or possibly even close to zero especially in regulated supplies) due to (small) gauge and length of wiring
Common mode noise attenuation due to twisting of wiring
Capacitance and inductance being added due to twisting of wiring (or using *ax cable) to remove noise

Test Equipment Leads
Resistance losses added due to the gauge and length of wire (coax vs twisted low AWG, resistance of meter also plays role)
Noise pick-up due to length of wiring
Common mode noise attenuation due to twisting of wiring
Capacitance and inductance being added due to twisting of wiring (or using *ax cable) to remove noise

This isn't apples vs. oranges - this is Granny Smith vs Red Delicious

[Richard Crowley]

Resistance losses added due to the gauge and length of wire (coax vs twisted low AWG)

Since nobody uses coax for power wiring this is rather a straw-man argument.

Power handling ability due to the gauge of wire

Which is one reason we don't use coax for power wiring.

Noise pick-up due to increases in (power supply) resistance due to (small) gauge and length of wiring

Noise is not picked up because of series resistance. 
As was already pointed out, power supply circuits are among the LOWEST IMPEDANCE of all possible circuits are are virtually immune from noise pickup.

Common mode noise attenuation due to twisting of wiring
Capacitance and inductance being added due to twisting of wiring

Since power supply circuits are such low impedance (and very will filtered on top of that) twisted wiring is of no benefit.

Test Equipment Leads
Resistance losses added due to the gauge and length of wire (coax vs twisted low AWG, resistance of meter also plays role)

Test equipment has extraordinarily HIGH impedance, so at the other end of the spectrum, series resistance of the test leads has no effect on the measurement.

Noise pick-up due to length of wiring

For sensitive AC measurements (or very low-level DC measurements) we use shielded wiring (coax or even triax cable).

Common mode noise attenuation due to twisting of wiring
Capacitance and inductance being added due to twisting of wiring

As has already been observed, twisting test leads is very rare because it is rarely convenient physically, and would be improper use of the equipment.

This isn't apples vs. oranges - this is Granny Smith vs Red Delicious

It is rather more like apples vs. shoes.

Power wiring is typically extraordinarily low impedance and copiously filtered. No benefit from shielding or even twisting.
Test lead wiring is typically extraordinarily high impedance and we use proper shielding where needed for noise mitigation.

[nukie]

I make my own leads, my power leads are made out of 18 awg multi strand super flexible silicon cable with 28 awg sense wire running along side to the very edge of the crocodile clip. So there's 4 wires and two types of gauge. Twisted pair test leads doesn't work for me, I use crocodile clip for ground and sharp test point for positive.

Maybe it works for 4 wire kelvin connection or SMD test tweezers that's about it. It's annoying to wire heavy gauge wires when you want to probe some logic connections.

[T3sl4co1l]

I always twist my cables 

For instance, I have some 18AWG pair coming up from my bench supply (a hand-built affair with +/-25V rails and enough iron, capacitance and darlington transistors to deliver 10A continuous or peaks over 50A... no, no current limiting ), which is actually some very nice foil screened cable.

It still has more than enough inductance that local bypass is a must (my breadboards usually end up with a 0.47 film and 220uF electrolytic).  But if it were just separate, loose banana jack cables, bleh...

If nothing else, a loose twist keeps the wires together so 1. you don't have so much mess and 2. you know where the current is coming from and where it must go (and if it's not going back that way, you have something to check!).

Tim