Safety Considerations – Operation with Earth-Grounded Cells and Auxiliary Apparatus
11
An earth ground connection can cause problems when testing batteries, fuel cells, or capacitors. Many of these
devices can source huge currents, often tens or hundreds of amperes. If the Reference 3000 chassis is earth
grounded, and another location in the stack is accidentally (or intentionally) connected to earth ground, a
portion of the stack is shorted through the Reference 3000’s cell cable. Very large current flows when this
occurs. Fuses in the cell cable will open up to prevent damage to the instrument. When this happens, the failed
fuses must be replaced before the instrument can be used again. The fuses in the cell cable are not essential for
operator safety. A section in Chapter 6 describes the fuses and their replacement in detail.
Operation with Earth-Grounded Cells and Auxiliary Apparatus
As described above, the Reference 3000 circuitry is isolated from earth ground, allowing it to make
measurements on cells that include an earth ground. This ground isolation is often called
floating
operation.
Cells with earth ground include many autoclaves, pipelines and storage tanks, and many fuel-cell systems.
Connection of the Reference 3000 to auxiliary apparatus will often earth-ground the Reference 3000,
destroying its ability to float and make measurements on earth-grounded cells. Connection of the Monitor
BNCs to an oscilloscope is an example where the instrument is earthed.
The
User I/O
connector can be connected to earth-grounded apparatus without earth-grounding the
Reference 3000, if the cable connections are done carefully. The metal shell on the Reference 3000
User I/O
Connector is connected to the instrument’s chassis, which is a floating ground. In a system that needs isolation
from earth ground, the shield of a
User I/O
cable must not connect the D-connector’s metal shell to earth
ground. Reference all
User I/O
signals to pin 6 of the D-connector, which is isolated ground on the Reference
3000.
The Reference 3000 contains surge suppressors that limit the voltage difference between the Reference 3000’s
chassis ground and earth ground to about 40 V. These surge suppressors are not part of the safety mechanisms
in the Reference 3000. Instead, they are present to limit the possibility of improper instrument operation or
instrument damage caused by electrostatic discharge (static electricity) and other surge events such as lightning.
Caution:
Floating operation of the Reference 3000 can be compromised by improper
cables to your I/O Connector. We do not recommend use of standard 15-pin shielded cables with this
connector. Custom cables with the shield connected to pin 6 of the D-connector are preferred.
The fuses in the Reference 3000 cell cable do not protect against a safety hazard. They are
needed to prevent damage to the instrument if it is improperly connected.
Warning:
Do not connect the chassis ground binding post to any voltage other than
earth ground. An improper connection can create a safety hazard, which could result in personal injury or
death.