Power connections for HP 6268B (208V hot vs. neutral)

[corrado33]

All,
I've been charged with getting a getting an old HP 6268B operational for my lab. It's wired for 208V (although it can be wired for 230/120 if so desired, as long as the plug is rated for it. However it would require a 24A 120V socket, so I can't easily switch it to 120 or else I would.)

However, I wanted to make SURE that it was wired correctly for 208, instead of the 230 it ships with (according to the manual). So I took it apart and made sure that all the taps were connected correctly etc. (They were.)

However, I've run into a problem. The 208V plug in our lab is a Nema 6-30. The plug that was on the power supply was a nema L6-30. (Essentially the locking version of the normal 6-30.) The 6268B calls for a ground, neutral, and hot. However, Nema (L)6-30 plugs are wired with 2 hots and one ground.

Anyway, I'm hesitant to just plug in the plug with the two hots because the manual says "make sure not to switch ac and acc" (hot and neutral). I mean, technically there is 208V between the two contacts of the plug regardless of whether you have two hots or one hot and a neutral. (Assuming it's wired correctly.) So, a voltage potential is a potential, regardless of how it gets "made"...

I can't rewire the outlet in the lab unless I want to pay facilities management a few thousand dollars to do it (yes, it's a rip off, I know, don't get me started.)

Anyway, would it be OK to just plug it in with the Nema  6-30 (and therefore, two hots)? I'm assuming the warning is in the manual so that you don't hook it up backwards. Is there anything I should be careful of?

[corrado33]

Turns out the 208V outlet (which is actually 215... weird) only measures 208 in one... uh... direction? (Polarization?) Therefore I can determine the "hot" from the... neutral? Haven't plugged it in yet, but we'll see if it works later.

I feel like an idiot talking about this. I've ever only dealt with 208/230 with my dryer at home (and they pretty much make it idiot proof to hook it up (green wire goes here... white wire goes here... etc.)

[XOIIO]

Turns out the 208V outlet (which is actually 215... weird) only measures 208 in one... uh... direction? (Polarization?) Therefore I can determine the "hot" from the... neutral? Haven't plugged it in yet, but we'll see if it works later.

I feel like an idiot talking about this. I've ever only dealt with 208/230 with my dryer at home (and they pretty much make it idiot proof to hook it up (green wire goes here... white wire goes here... etc.)

Just to be sure, you are measuring in AC right?

Anyways, black is live, white is neutral, and green is earth in north America, you can just google the standard wiring for your region.

[Monkeh]

Turns out the 208V outlet (which is actually 215... weird) only measures 208 in one... uh... direction? (Polarization?) Therefore I can determine the "hot" from the... neutral? Haven't plugged it in yet, but we'll see if it works later.

It is not weird that it's 215V. Measure it another time, it will change. Welcome to ohm's law, you should be familiar with it..

E: Confused crap removed.

[ConKbot]

208V is the line to line voltage of 208 delta or 208/120v star three phase. You should be able to measure 120v from ground to phase on each of the 2 phases, and also 120v from neutral (if present) to each of the phases.  If you phase-neutral measurements are off from one another by more than a handful of volts, there is a load balancing issue in your panel, but thats an electrician problem, not something for you to handle.   But measuring on AC from phase to phase, you should get 208 (ish ) volts with the meter either way.  Looking at the manual on agilent's site, youre just wiring the input up to the correct transformer taps, so it shouldn't be a problem connecting your 2 phase leads to the inputs.  Where are you seeing a reference to 'neutral' ?  The manual at http://www.home.agilent.com/en/pd-1000001533%3Aepsg%3Apro-pn-6268B/dc-power-supply?&cc=US&lc=eng  only shows "AC input" with 0v, 115, 208v, and 230v taps/wiring arrangements.  Even 230v would get 2 legs of the 140/120v split phase in the US, so I'd expect they weren't counting on the '0v' AC input to be anywhere near earth. 

[Monkeh]

Oh, right, that's your 3-ph system.. christ, I get confused, you guys can't pick a single configuration.

[corrado33]

Where are you seeing a reference to 'neutral' ?

Page 17 (3-1) under the "Caution" heading.

[corrado33]

Just to be sure, you are measuring in AC right?

Anyways, black is live, white is neutral, and green is earth in north America, you can just google the standard wiring for your region.

Yes, I am measuring AC 

Unfortunately I'm not actually allowed to remove the plug plate to see the wiring behind it. If someone caught me, our lab could be fined etc. bad stuff etc.

[ConKbot]

Where are you seeing a reference to 'neutral' ?

Page 17 (3-1) under the "Caution" heading.

Strange, both should be equally isolated, and the warning definitely enough to cause pause. Perhaps its more to do with the switching the unit on and off, as you dont want to break the neutral and leave the phase floating, though both would be equally effective, one leaves live mains around inside the chassis.

[Monkeh]

Where are you seeing a reference to 'neutral' ?

Page 17 (3-1) under the "Caution" heading.

Strange, both should be equally isolated, and the warning definitely enough to cause pause. Perhaps its more to do with the switching the unit on and off, as you dont want to break the neutral and leave the phase floating, though both would be equally effective, one leaves live mains around inside the chassis.

Most likely just an assumption of single-phase supply, where switching and protective devices would be installed on the line rather than neutral. In practice both line and neutral are considered live conductors and require equal insulation.

[corrado33]

Strange, both should be equally isolated, and the warning definitely enough to cause pause. Perhaps its more to do with the switching the unit on and off, as you dont want to break the neutral and leave the phase floating, though both would be equally effective, one leaves live mains around inside the chassis.

Exactly the reason I paused and asked the "experts" on the internet. 

Anyway, here's twice I feel dumb.

Turns out the "blades" for the Nema 6-30 are very long... The connections in the socket itself are very far back, almost out of reach of my multimeter probes. Therefore when I said it only showed 215 in one "direction," I was mistaken. I thought that seemed weird.

You guys are correct, both legs measure 120ish to ground. (And therefore by vector addition we get 208 when we measure between them!)

Anyway, plugged it in and started it up and all is well. The thing works great! Thanks for the help!