Suppose you have 120v going to a load. How come when you measure from hot to chassis, you get 60v? Half. I never quite understood this.

 

Suppose you have 120v going to a load. How come when you measure from hot to chassis, you get 60v? Half. I never quite understood this.

It sounds to me like you have something running off of a "centre tapped transformer". Here in the UK, these are used extensively on building sites to power tools.
Your hot wire is at +60V but your cold wire is at -60V. As far as the load is concerned, it's connected to a 120V supply as there is 120V across hot and cold. If you were to measure from hot to Ground or chassis, you would only read 60V.

It is vital to remember that Voltage is a RRELATIVE measurement. Hot is at 120V RELATIVE to cold but it is only at 60V RELATIVE to Ground.

 

Suppose you have 120v going to a load. How come when you measure from hot to chassis, you get 60v? Half. I never quite understood this.

Sounds like a serious problem to me. The chassis should be connected to the ground wire and be electrically isolated from the hot line. In the US, chassis ground should also be isolated from neutral. The only place ground and neutral are supposed to be connected is in the circuit breaker box.

 

In some cases this is a confusing, but normal reading when measuring to ground from isolated mains powered equipment, when using modern high impedance meters, you are measuring a capacitive/inductive leakage to ground.
IF the equipment is supposed to be earth grounded - mains energized equipment, there could be a problem, e.g. where equipment that is NOT double insulated and does not require a earth ground..
Max.

 

It could be balanced power but more likely it's like what @MaxHeadRoom said. 120vac balanced power is rare in a US residential environment but it sometimes used in a critical measurement controlled environment.
http://www.mikeholt.com/mojonewsarc...7-Sensitive-Electronic-Equipment~20050102.php

 

Add a 10kΩ resistor between hot and chassis and see if the voltage changes.

 

I've seen this in some "budget" power bricks and it is often caused by the mains capacitive filter. There is a capacitor from each mains line, sometimes connected to the output -ve line and without an earth, this gives a half voltage at fairly high impedance with respect to ground. Not real good as it can have enough current capability to damage some parts or give you a bite! Considering here in Oz, the mains is 240V, so 120VAC can be measured to ground.

 

I don't know the theory behind this, perhaps another member can help, but I work on a medical laser with a high voltage power supply that is isolated from the laser chassis because of electromagnetic radiation. There is no earth connection to the power supply and it's enclosure is held at half the input mains potential.

Does your device have the potential to emit high levels of electromagnetic radiation?

 

Add a 10kΩ resistor between hot and chassis and see if the voltage changes.

If the voltage is real, even a 10k resistor would draw enough current to heat up quickly, wouldn't it? I come up with 360mW, unless I just did the math wrong.

 

If the voltage is real, even a 10k resistor would draw enough current to heat up quickly, wouldn't it? I come up with 360mW, unless I just did the math wrong.

The reason for a test such as this is to see if it sustains the current, IOW, does the voltage collapse when a lower impedance path is offered?
If so, it suggests a reading due to the high impedance meter, possibly from the inductive or capacitive means.
Max.

 

The reason for a test such as this is to see if it sustains the current, IOW, does the voltage collapse when a lower impedance path is offered?
If so, it suggests a reading due to the high impedance meter, possibly from the inductive or capacitive means.
Max.

Right, but if not, you better be using a fat resistor, or else you won't want to leave it turned on very long!

 

Suppose you have 120v going to a load. How come when you measure from hot to chassis, you get 60v? Half. I never quite understood this.

Under this condition, you would find the other half from neutral to chassis!
The problem is due to earth not connected.
The emi filter's common mode filter has 2 caps, one between line and the earth, and the other between neutral and earth, and this is connected to the chassis. If the equipment is not connected to the mains earth, these 2 caps act as potential divider, and the chassis is at half the voltage of mains.