Hysteresis The hysteresis determines the level or volume at which the relay re-energises after the
trip point has been exceeded or undershot. It is expressed as a level or volume and is
entered at V1H2 for relay 1 or V1H7 for relay 2:
Step Matrix Entry Significance
1 V1H2 e.g. 400 Hysteresis for relay 1
(in the unit displayed in field V0H0)
2 - »E« Register entry
Relay at fault When the relays are assigned to channel 1 and the self-monitoring circuit triggers on a
fault, relay 1 responds according to the entry at V1H3 and relay 2 to the entry at V1H8:
• 0 = de-energised
• 1 = as analogue output, i.e. –10%, +110% or hold
depending on entry at V0H7.
Step Matrix Entry Significance
1 V1H3 e.g. 0 The relay 1 de-energises at fault
2 - »E« Register entry
With option 1, the behaviour of the analog output, which was preset in V0H7, determines
the reaction of the relay to a fault. Table 7.3 summarizes the possibilities:
7.2 Applications
Depending on the hysteresis settings, several operating modes are possible for
continuous measurement at channel 1:
• limit switch
• two point operation with one relay
• two point operation with two relays
Limit switch If the relay is to act as a limit switch only: Set the hysteresis at V1H2 or V1H7 low or to 0.
The relay trips when the level of volume drops below or rises above the switch point.
When the level or volume returns to normal, the relay energises almost immediately.
Step Matrix Entry Significance
1 V1H0 e.g. 10 Switch point = 10
2 - »E« Register entry
3 V1H1 0 Minimum fail-safe mode
4 - »E« Register entry
5 V1H2 10 Hysteresis = 10, the relay energises again at level = 20
6 - »E« Register entry
Setting at V0H7 Minimum fail-safe mode Maximum fail-safe mode
0 = –10% (–2 mA) Relay de-energises Relay energises
1 = +110% (+22mA) Relay energises Relay de-energises
2 = hold (last value) No change No change
Table 7.3:
Relay response on fault condition.
Silometer FMC 671 Z/676 Z Chapter 7: Limit Switches
38
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