307UM0100_01 May 2016 PD17x User Manual Page 13 of 31
4.5 Powerfail
The PD17x is designed with a Powerfail feature that allows it to remember its operating conditions and detect status in
the event of power being removed. When power is restored, the unit continues operating from its saved conditions and
status. This is designed specifically for fail safe situations to retain the output state and prevent a glitch on the outputs
for a power failure. As such, when the power is restored the detector will not retune but will instead return to the detect
state prior to the power failure. If a vehicle was on the loop during the power failure, it will remain detected when the
power is restored. Thus it prevents the tuning out of a vehicle over the loop during a power failure.
The memory retention of the vehicle is designed to be infinite, but is subject to the following limitations:
The level of the detect must be sufficiently greater (at least 0.8%) than the sensitivity level or else the maximum
potential temperature drift from one time of day, when the power fails to another when the power returns, mustn’t
exceed 20°C.
The level of the detect should be no less than 0.15% and a minimum of 0.8% above the sensitivity level.
With AFS activated, none of the channels must be in fault since this will cause the unit to retune on recovery.
If the presence of a vehicle should occur less than a second before power is lost, there is a chance that the unit won’t
retain the detect and the outputs could toggle on restoration of power.
If these conditions are not met, it is possible for the detect condition to be lost on recovery of power.
Powerfail cannot be guaranteed to work if a user interferes with the detector in any way when the power
is OFF. No settings are allowed to be changed. The enclosure housing of the detector shall NOT be opened
when the unit is OFF and Powerfail is turned on, as any drastic change in temperature could result in
Powerfail starting up in the incorrect state. Nortech accepts no responsibility or liability in the event of
incorrect behaviour caused by user interference.
4.6 Automatic Frequency Selection
AFS allows the detector to briefly evaluate all five frequency bands and select the best operating frequency available. It
weighs up each selection based on where the frequency is located within the operational range, the signal strength, and
the level of detected noise. AFS allows the detector to evaluate all the frequency selections.
Due to the increased processing required, AFS takes longer to tune than when it is deactivated. The tune time with AFS
on can range from 5 to 20 seconds. If after this period of time, the detector still has not tuned, it is also possible that
none of the frequency selections are suitable for the loop. If this occurs, manual frequency selection should be used.
Due to the sporadic nature of noise, the channel may seem quiet during the evaluation but still suffer from cross-talk.
AFS can be toggled on or off via the DIP switch. While the detector is tuning, it is recommended to allow the AFS tuning
to complete.
On both manual and automatic frequency selection, once a valid frequency offset has been selected, the detector waits
for that setting to settle below the sensitivity setting before allowing normal operation to continue. If there is sufficient
drift from an extreme temperature change in the installation location or vehicles driving over the loop at the time, it is
possible that the tuning will timeout, invalidate that frequency selection and attempt to tune again. If the situation is
very close to the limit, it is possible that it might tune as the drift or noise goes back over the threshold and the unit
goes into detect just as it tunes. In a noisy environment, this can be prevented by retuning and if the situation persists
to decrease the channel sensitivity.
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