Hioki Super megohm SM7110 - Page 178

Appx.10
Assembling Switching Unit for Objects to be Measured
Selecting relays to be installed in the switching unit
Important specications for relays
Switching voltage and dielectric strength
“Maximum switching voltage” and “dielectric strength between contacts and between a contact and a coil”
must be su󰀩ciently high with respect to the instrument’s set voltage.
Switching current
Select relays with switching currents that are high enough relative to a current owing even if the terminals of
a measured object are short-circuited with each other.
In addition, the rated maximum current of the relays must be higher than a transient surge current, which is
generated at the moments when contacts become closed and open.
Thus, generally you should include a resistor that limits the current in series with the relay as close to as
possible.
Insulation resistance
An insulation resistance between contacts and that between a contact and a coil must be high enough to be
negligible with respect to a insulation resistance of a an object to be measured (1000 times greater or more).
For example, if the insulation resistance of the object to be measured is 100 M
Ω
, use a relay with an insulation
resistance 100 times greater or more (10,000 M
Ω
or more) than the insulation resistance of the object to be
measured. (If 100 times greater, the error from connecting the switching device to the instrument will be 1% of
a measured value; and 1000 times, 0.1%.)
Shape, structure
An ideal structure of a relay is such that its contacts are pulled out with a lead wire, and the contacts and coil
are shielded with each other.
For a plug-in type relay, an insulation resistance between pins installed in a relay socket themselves are
included in parallel with a space between contacts in the relay, resulting in the negative e󰀨ect that measured
insulation resistance values are lower than expected.
In addition, for the plug-in type relay, long-term use leads to the socket being covered with dirt due to the dust
attraction e󰀨ect of high voltage, and causes insulation deterioration.
Contact leads
Coil pins
Shield
(guard)
Example (measurement voltage is 350 V or less)
Manufacturer: Sanyu Electric, Inc.
Model: URM series
Maximum switching voltage: 350 V
Maximum switching current: 0.5 A
Insulation resistance between contacts: 10
13
Ω
or 10
14
Ω
can be specied
Between contact and guard: Same as above
Between coil and guard: 10
11
Ω