multitap ballast, lamp cord voltage?

Status
Not open for further replies.

jake357

Member
I have a question. If you have a multitap ballast and wire it for 120 or 240... Doesn't matter. Will the conductors of the lamp cord from the secondary side always see the same voltage and current path?

I guess I don't know hoe to word it properly.

If the primary side operates on 240v, will the lamp cord still only have current going to the center contact of the socket? Or does the shell now somehow have 120v along with the center contact of the socket.

Probably a really stupid question.

Am I right in assuming that it does not matter what the primary side runs at as far as voltage, the secondary side will always output the same voltage and current and the conductors of the lamp cord will always remain the same? As in the center contact of the socket is hot and the shell returns the current back to the ballast?

Thanks and sorry for the probably stupid question. I'm a telcom guy and I was discussing this with a co-worker yesterday....
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
This thread had been temporarily closed, as it appeared to be an attempt by a non-licensed electrician to obtain assistance with an electrical installation project. In an exchange of private messages, I have been assured that this is not the case. The discussion is an academic one, and is being debated among several electricians working for the same company as the OP.

I offer an apology to the OP for the delay and the inconvenience.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
From the perspective of the light bulb, there is no difference between wiring the balast for 240 volts and supplying it from a 240 volt source, and wiring the balast for 120 volts and supplying it from a 120 volt source. The differences between the two wiring methods are internal to the balast, and the output of the balast is the same in either case.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
:) Welcome to the Forum, Jake357.

If the ballast is magnetic, then each tap in a "multitap" ties to different points on the primary winding. That is, the transformer turns ratio is different for different input voltages.

If the ballast is not magnetic, there are more answers, but the generalization that Charlie just gave holds exactly true.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Many multitap ballasts directly connect one input terminal to the lamp, and control the other side of the circuit to the lamp.

What this means is that one lamp terminal is at whatever the supply leg voltage is, and the other lamp terminal is at whatever the ballast makes it relative to the first terminal.

No matter which input voltage is selected, the lamp will see exactly the same voltage between its terminals, and the lamp will operate properly if a correct supply voltage is used.

The internal voltages of the system do not depend upon the supply voltage, nor do they depend upon the grounding of the supply conductors.

Where the grounding of the supply conductors makes a difference is in the voltages relative to the outside world.

See these diagrams:
http://www.venturelighting.com/BallastHTMLDocuments/wiring_diagrams-Stand.html

The screw shell of the lamp gets connected to the 'white' wire coming out of the ballast. If this white wire is connected to a grounded circuit conductor, then the screw shell will be at roughly ground potential. If this white wire is connected to a hot circuit conductor, then the screw shell will be at the hot conductor voltage. The ballast will maintain the proper _relative_ voltage between screw shell and center pin, and the lamp will function properly, but the ballast does nothing to change the voltage between screw shell and ground.

Depending upon the installation, it might be a violation to have the screw shell at an ungrounded voltage (I'm tempted to say that this is always a violation, but I'm not sure; I know that in residential installs the screw shell is supposed to be connected to the grounded conductor only.)

-Jon
 

jake357

Member
Ok... So you will have voltage on the socket shell and not just the center contact regardless of the primary voltage?
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
I don't quite follow that question. Voltage does not "exist" at any single point. Voltage exists between two points. When you turn on the light, there is voltage between the shell and the center contact. That value of voltage will be the same, regardless of how you wired the balast, provided of course that you wired it in accordance with the manufacturer's instructions. When you turn off the light, there will still be a value of voltage between the shell and the center contact, but the value will be zero volts.
 

jake357

Member
Correct. I guess what I'm trying to say is, if the secondary outputs somewhere around 500v, and the primary runs at 240v, then when you measure from center contact to a known ground, will you get 500v or do you get your 500v by measuring between the socket shell and the center contact?

The general question between us was, should you switch the socket shell or just the center contact? I guess neither one of us understands the voltage potential between both conductors of a lamp cord that powers a HID bulb.
 

jake357

Member
It would probably be helpful to give the context, which is what I should have done in the first place. I was at an office building running more fiber back to a warehouse type space in the back of the building.

They had a board next to the panel that controlled all there high bay lighting. Judging by the spectrum I'm guessing they were metal halide. What they were doing is plugging the ballasts into their own receptacles primary side @ 120 (120/208 wye), then they were controlling most of the ballasts with DPST relays line side with a cheapo timer that daisy chained between all the center contacts. But 2 of the ballasts were wired with DPDT relays and after tracing the lamp wire, one set ran out to the back of the building to fire a hid light on the side of the building.

So after looking at it for awhile, we decided that the NO contacts were feeding the outside HIDS. And when the timer came on in the AM, the DPDT would close and then fire up one of the high bays in the warehouse whilst turning off the outside HID.

Kind of clever, but our debate was, It's a 120v source primary side but they were switching everything secondary side of the ballast (via relay). Were they switching a neutral in the lamp cord that ran up to the hid, or do both wires have a voltage other than 0 on them.

So then we got to arguing what if the primary side was 240... What happens to the conductors on the lamp cord then? both conductors carry voltage other than 0?

My co-worker argued the neutral was being switched with that DPDT relay. Is it really a neutral once it's past the secondary side of the ballast?

:confused: I hope I didn't loose anyone besides my self... ;)
 
Last edited:

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I think that charlie b is hitting a key misunderstanding.

Jake: voltage is always measured between two points. You have to connect the two probes of the volt meter to two things, and the meter will tell you the voltage difference between those two points.

No matter what the nominal input voltage is, the job of the ballast is to control the voltage between the screw shell and the center tap to be whatever the lamp requires. (Note: because the ballast is acting to both start the lamp and control lamp current, this voltage is _not_ a constant, but changes depending on lamp age, operating state, etc. But the ballast is acting to control the voltage to the necessary value at that moment.) In order to measure this voltage, you touch one meter probe to the center tap, and one to the screw shell. (Disregard how to actually make this connection; we are talking theory here.....)

It won't matter if the ballast is supplied with 120V H-N or 208V H-H or 240V H-H or 240V H-N. The ballast (if rated for the input voltage) will control the voltage between screw shell and center tap.

A _separate_ question is 'what is the voltage between the screw shell and ground'? In other words, if I touch one volt meter terminal to some grounded metal (say the EGC terminal at a receptacle) and touch the other meter terminal to the screw shell, what voltage will I read. The answer to this question _will_ change depending upon how the supply is connected. If you supply the ballast with 120V H-N, connected properly, then the voltage between the screw shell and ground will be very low. If you supply the ballast with 208V H-H, then the voltage between the screw shell and ground will be about 120V.

In either case the voltage between the screw shell and the center terminal is the same, but the voltage between these terminals and ground will be different.

-Jon
 

jake357

Member
If you supply the ballast with 120V H-N, connected properly, then the voltage between the screw shell and ground will be very low. If you supply the ballast with 208V H-H, then the voltage between the screw shell and ground will be about 120V.

O.K... I think that answers my question. As far as those 2 DPDT relays are concerned, they were hooked up correctly by switching both conductors in the lamp cord because neither one of them will measure a voltage of 0 to ground, therefore for safety purposes, you need to switch both conductors going out to the socket assembly, otherwise if you happen to be grounded on some hvac next to the fixture and touch the socket shell, your still going to get a jolt even tho the primary of the ballast is powered by 120v?

That was the main question both of us had. Whether or not you could throw a SPDT in the place of that DPDT and just wirenut the neutrals and switch the incoming and outgoing hot.

Sorry to drag this out. I understand about voltage potential. Just wanted to know if it was a safety hazard if there was a SPDT in there instead of a DPDT for the 2 ballasts that were firing outside lights as the rest of the inside lights went off....
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
Whether or not you could throw a SPDT in the place of that DPDT and just wirenut the neutrals and switch the incoming and outgoing hot.
Sure.

You can.

Get out your Code book and look up the definition of Premises Wiring (System) and Utilization Equipment. Article 100 Definitions.

The NEC covers the Premises Wiring. The wiring internal to the luminaire is not part of the Premises Wiring.

Safety is another issue, beside the question that you are asking. It sounds like you assume the DPDT 240 V relay controlling two separate 240 V circuits (by switching one leg only of each 240 V circuit) is "unsafe" and therefore can't be done. If there are disconnecting means in the Code required spots, those disconnecting means provide the safety required . . . not whether there is a control relay somewhere that is doing something else.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
Here's a parallel.

A 240 V electric wall heater in a home is controlled by a wall mounted line voltage thermostat.

The common thermostat, in operation, only switches one side of the line of a 240 V supply to the heater, leaving the heater "hot" even when the thermostat is not calling for heat.

In operation, the thermostat is the control relay, switching only one energized leg.

The safety required for servicing is in the shut off built into the thermostat when one turns it all the way down. That shut off disconnects both energized conductors supplying the heater.

Now, the disconnect is not required to be at the thermostat . . . it just happens that that is a cheap way to do it.
 

jake357

Member
Thanks for the input... Even tho a DP T-stat can't be considered as a disconnecting means because you can't lock it in place, I still use them anyways vs a SP t-stat on a 240v baseboard heater just so you can service them without throwing the breaker. I''m familiar with disconnecting means and requirements.

These ballasts are fed with 120v on a 120/208 wye system however.

The DPDT relay has 120v coil contacts which are energized by a 120v timer roughly coordinated to coincide with the hours of light outside. The relay switches the secondary side of the ballast but the primary side is fed with 120v.

If I could just measure between both conductors of the lamp cord and a known ground I could answer my question without all the confusion.

This is about as much as I can narrow my question down:

Should you ALWAYS switch both conductors of a lamp cord secondary side of a ballast for safety purposes regardless of what the input voltage is?


Regardless of what the primary input voltage is, will you always get a difference in potential between the shell socket and ground up at the fixture?

Thanks guys for bearing with me. I'm just half tempted to take a 600v meter with me tomorrow and measure the load out contacts of that DPDT. It will answer the question that I'm seeking, altho I really have no business going near it seeing as how I'm there to run fiber....

Thus, the reason I'm trying to ask here. :D
 
Last edited:

jake357

Member
Truely remote. Power source, remote ballasts and relays are all on a board and in enclosures next to the panel. EMT comes from the relay enclosure, up the wall and over to the reflectors. Hits a Junction box and a whip connects.

We are talking a good 50 feet or more. I assumed that's why they were using such large conductors. I also wouldn't be surprised if the capacitor has been moved up to the hoods...

Anyway..... Seems we are kind of fizzling out here. I'll just take a reading and be done with it tomorrow. XFMR's were never my thing... Just trying to understand is all.

I did think the on/off transfer between the outside and inside lights was pretty cool. Those 2 ballasts run 24/7 but you can see the arcing soot on the DPDT's....

The other high bays are on DPST's. All of this lighting controlled by one little timer. Kinda neat.
 
Last edited:
Status
Not open for further replies.
Top