Grounding NI USB 6343 DAQ device with chasis ground lug

[Mr_Engineer]

1. With reference to X series user manual (Page # 17); I want to connect chasis of NI 6343 DAQ device with building ground as mentioned in the user manual.

2. There is a potential difference of 5V between neutral and building earth, idealy it has to be zero.

3. Is it electrically safe for NI USB 6343 DAQ device to be connected with that building earth which has a potential difference of 5V with respect to neutral input?

4. How much Potential difference between neutral input and building earth is acceptable by National Instruments, which will not cause any demage to DAQ device?

Kind Regards

Jamal

[johnsold]

I would be very careful about connecting any sensitive (or  expensive) test equipment to the power system in that building.

That much potential difference implies that a significant amount of current is flowing in either the neutral or the ground. If it is in the neutral where it is supposed to be, it may indicate a bad connection or excessive current for the size and length of the conductors. If it is in the ground, the grounding connection may be very poor with just a small amount of current or a normal connection may be carrying a lot of current (which suggests the possibility of a ground fault). The cause should be identified and corrected. 

Any of these conditions can lead to unexpected currents flowing through ground conductors on your  DAQ device or computer. USB cables in particular are not designed to carry very much current.  Even if those ground currents do not damage the DAQ device, they may cause erroneous measurements.

This is not intended to be specific answers to the questions in 3 and 4 of your post. The answers for your situation will depend on the details of how everything in the system is wired.  For example exactly where and how many places are neutral and ground connected? What is the capacitive and inductive coupling between grounding conductors, neutral conductors, and other energized conductors? This may include equipment physically nearby but not actually connected to the system.

Lynn

[Mr_Engineer]

1. Thank you so much for kind response.

2. When I turn on electrical appliances including computers and other electronic equipment; a potential difference of upto 5 Volt ac appears between Neutral and building earth. When All equipments are OFF; there isn't any potential difference between  neutral and earth inputs.

3. I have red complete user manual of X series DAQ devices but I didn't come up with some specific explaination whether to connect DAQ device with that building earth which has a 5 Volt P.D with respect to neutral input of A.C mains since X series devices are electrostatic sensitive (Ref. to page # 17 of X series user manual) and OEM recommends that these devices must be connected with building earth in order to avoid any electrostatic demage to them.

4. Could you or anyone else from NI technical support team can provide any justified answer about my queries related with earthing of X series devices in context of user manual page # 17?

5. Note that there isn't any electrical connection between neutral and earth inputs of AC mains since earthing is used to safeguard any accidental short circuit inside any electronic equipment connected with earth and provide a low resistance path to the leakage current which in other case may damage that equipment or may cause personal injuries.

Kind Regards,

[johnsold]

In the United States electrical practice calls for neutral to be connected to earth at the building service entrance (and nowhere else). If I recall correctly the electric code allows up to 10 volts between neutral and ground but that is expected to only occur under fault conditions. 

Your item 2 suggests to me that the neutral wiring is marginally sized for the loads or has a poor connection.  The potential difference is likely due to the current flowing through the impedance of the neutral conductor.  How much does the line voltage at the farthest point from the service entrance vary between no load (all equipment off) and when all the devices are on?

I do not have any knowledge of the internal wiring of the device beyond what is in the manuals.

Lynn

[Mr_Engineer]

1. Voltage between phase and neutral remains constant but P.D between earth and neutral inputs varies from zero to 5 Volts when no equipment are powered and when all equipment are ON.

2. Note that Neutral and Earth are not interconnected at any point.