Making Connections To The Multimeter -- General Guidelines - Fluke 8508A User Manual

8508A
Users Manual

Making Connections to the Multimeter -- General Guidelines

The 8508A Reference Multimeter is capable of providing highly accurate traceable
measurements. To attain this accuracy, it is necessary to make connections to any
external circuitry or load correctly. A few guidelines are given in Table 3-1.
Thermal EMFs
These can give rise to series (Normal) mode
interference, particularly where large currents
have a heating effect at junctions.
In otherwise thermoelectrically-balanced
measuring circuits, cooling caused by drafts can
upset the balance.
E-M Interference
Noisy or intense electric, magnetic and
electromagnetic effects in the vicinity can disturb
the measurement circuit. Some typical sources
are:
• Static electric fields.
• Fluorescent lighting.
• Inadequate shielding, filtering or grounding of
power lines.
• Transients from local switching.
• Induction and radiation fields of local E-M
transmitters.
• Excessive common mode voltages between
source and load.
The disturbances can be magnified by the
capacitance of the user's hand. Electrical
interference has greatest effect in high
impedance circuits. Separation of leads and
creation of loops in the circuit can intensify the
disturbances.
Lead Resistance
The resistance of the test leads can drop
significant voltages between the source and
load, especially at high load currents.
Lead Insulation Leakage
This can cause significant errors in measurement
circuits at high voltages and high resistances.
Some insulating materials suffer greater losses
than others, e.g. PVC has more leakage than
PTFE.
3-4
Table 3-1. Identifying and Avoiding Errors
Sources of Inaccuracy Avoiding or Minimizing Inaccuracy
• Screen thermal junctions from draughts.
• Allow time for thermal equilibrium to be reached
before taking readings.
• Use conductors, joints and terminals with a good
margin of current-carrying capacity.
• Avoid thermoelectric junctions where possible:
∗ Use untinned single-strand copper wire of high
purity.
∗ Avoid making connections through Nickel, Tin,
Brass and Aluminum. If oxidation is a problem,
use gold-plated copper terminals, and replace
the terminals before the plating wears off.
∗ If joints must be soldered, low-thermal solders
are available, but crimped joints are preferred.
∗ Use low-thermal switches and relays where
they form part of the measuring circuit.
∗ Balance one thermal EMF against another in
opposition, where possible. (Switch and relay
contacts, terminals etc.)
• Choose as "quiet" a site as possible (a screened
cage may be necessary if interference is heavy or
circuit impedance is high). Suppress as many
sources as possible.
• Always keep interconnecting leads as short as
possible, especially unshielded lengths.
• Run leads together as twisted pairs in a common
shield to reduce loop pick-up area, but beware of
leakage problems and excessive capacitance.
• Where both source and load are floating, connect
Lo to ground at the source to reduce common
mode voltages.
• If an external measurement ground connection has
been made, select External Guarding at the
Multimeter in Voltage and Current functions and
deselect External Guarding in Ohms and PRT
functions.
• When connecting to a Multifunction Calibrator such
as the Fluke 5720A or 5520A, follow the above
Guarding and Grounding advice and deselect
External Guard at the Calibrator.
• Keep all leads as short as possible.
• Use conductors with a good margin of current ­
carrying capacity.
• Use External Guard or 4-wire connections where
necessary.
Choose low loss insulated leads - PTFE is preferred
to PVC. When running leads together in shielded
pairs, avoid large voltages between leads in the same
screen, especially if using PVC insulation.