400 Hz neutral/ground

Status
Not open for further replies.
A

arclightzero

Member
I have a couple of questions regarding 400Hz power. At several points along our bus we have distribution boxes that have the 400Hz neutral line tied directly to our safety ground. Obviously, when ohming out the system, I have neutral and groud at the same potential. Given how many times this is occuring, is 400hz distribution different than 60Hz? I know that the NEC calls for bonding only at the transformer, bit I couldn't find anything that was referenced to 400Hz. Anybody ever seen anything like this?

[ March 17, 2004, 08:47 PM: Message edited by: arclightzero ]
 
Re: 400 Hz neutral/ground

What is the capacity of this 400 Hertz system?

Give circuit breaker size. Voltage.
 
Re: 400 Hz neutral/ground

Same rules apply for 400Hz. If you are finding a multi-grounded neutral, you have a code problem.

The two main differences between 400Hz systems and 60Hz systems is the size of the transformers and voltage drop. 400Hz transformers are smaller for a given KVA size, and there is more voltage drop due to the higher frequency.

Typically you find 400 Hz systems on aircraft, military, and some high end computer mainframes.
 
Re: 400 Hz neutral/ground

Originally posted by bennie:
What is the capacity of this 400 Hertz system?

Give circuit breaker size. Voltage.
Click to expand...
Our boxes are rated at 600v and 100 amps. I am not entirely sure off the top of my head as to the rating of the breakers, but It's a pretty beefy system... I'm pretty sure they're rated at the full 100 amps.

[ March 17, 2004, 10:07 PM: Message edited by: arclightzero ]
 
Re: 400 Hz neutral/ground

This appears to be a factory manufactured system.

Is the 400 Hertz from a MG set? The redundant grounding may be for lowering the loop impedance.

This is apparently a specialized item of equipment for a specific purpose. The NEC may not be involved in the design critereon.
 
Re: 400 Hz neutral/ground

As far as I know, the system isn't too highly specialized. We have a 400Hz bus that simply distributes power out from the transformers to our shops. On the bus we have breaker enclusures that provide lines to 400Hz outlets, 28VDC rectified outlets and various test benches. The bus is fairly generic in that it provides to all of these different things. If I can figure out a way to post pictures to this, I have some shots of our trouble spots.
 
Re: 400 Hz neutral/ground

What are you testing?

28 volts suggest aircraft accessories.

600 volt, 400 Hertz is unknown to me.

[ March 18, 2004, 08:15 AM: Message edited by: bennie ]
 
Re: 400 Hz neutral/ground

Sorry, I didn't mean to imply it was a 600 volt system. It's a 120vac system, our boxes are rated to 600 volts, but we're nowhere near there. The breakers that we have vary from 30 amp on up.
You are right though, this is for aircraft parts.
 
Re: 400 Hz neutral/ground

The 115/200 volt, 400 Hertz power for aircraft is wye connected, with the star point being connected to the airframe itself.

Only three wires are connected to each gyro instrument stator. The gyro motor is delta wound.

I began my career in aircraft work. That was over 50 years ago.

To answer your original concern about the neutral ground being interconnected, the star point is actually a ground point.
 
Re: 400 Hz neutral/ground

arclightzero, I imagine some electrician had a high voltage developed between N-G, and to solve the problem he/she violated code by shorting the neutral and ground bus down stream of the M-G, UPS, or converter rather than address the real problem of the conductors being undersized for 400Hz applications.

The same rules apply to premises wiring on 60 and 400Hz systems. However there is a voltage drop design consideration on 400Hz systems. You can use the minimum wire specified in 310.16 but if your circuits are longer than say 30 feet, you are going to experience excessive voltage drop at 400Hz. The solution is oversized circuit conductors, not shorting the neutral and ground buses down stream.

Not only does shorting the neutral and ground buses violate code, but it will also introduce common mode noise which can certainly interfere with instruments used for calibration. Not something you want in aircraft instrument calibration.

[ March 18, 2004, 10:42 AM: Message edited by: dereckbc ]
 
Re: 400 Hz neutral/ground

Thanks for the help! I'm going to take some pictures and document some of this stuff and talk to the building engineers about it.
 
Re: 400 Hz neutral/ground

One question I do have is if this sort of thing is a safety hazard to personnel, or is it simply bad for the equipment? I am having a difficult time finding hard documentation on risks associated with this.
 
Re: 400 Hz neutral/ground

It is a safety hazard as well as causing problems like the common mode noise Dereckbc mentioned.

The idea behind bonding the neutral and ground at only one location is to prevent current from flowing through the equipment ground conductor during normal operation. When a second bond is added, current flows from the source along the hot conductor and to your equipment. Then, the current takes multiple paths back to the transformer. Some of it flows back on the neutral like it should. Some flows back on the equipment grounding conductor, and some may flow through the building steel, metal conduits, metal cases of panels, etc.

Having these paths creates noise, and worse, it causes a voltage drop along the path. The result of the voltage drop is that things that are supposed to be at 0 volts (like the metal case of the equipment you are testing) are really at several volts potential.

Steve
 
Re: 400 Hz neutral/ground

Originally posted by arclightzero:
One question I do have is if this sort of thing is a safety hazard to personnel, or is it simply bad for the equipment? I am having a difficult time finding hard documentation on risks associated with this.
Click to expand...
It is both a safety issue (Code) and operational issue. From the safety point of view you are intentionally putting load current on a fault clearing path, the EGC. It can impede the operation of OCPD's, and develop voltage on metallic objects where there should be none.

Operationally the current develops voltage differences along the ground circuit refered to as common mode noise. If you are using the power safety ground (EGC) with test equipment, A/V, data. etc that use the EGC as a signal reference, then it can iduce errors or noise.

Try IEEE Emerald book for reference material. There is a lot of info on the subject, but most will set you back a few dollars.
 
Status
Not open for further replies.
Top