SST (Pakistan Educators Recruitment Tests) cheat sheet

Ohm's law: V = I × R. Power: P = V × I = I² × R.

Series circuits: RT = R1 + R2 + … and the same current flows through every element. Parallel circuits: 1/RT = 1/R1 + 1/R2 + … and the same voltage appears across every branch.

A normally closed (NC) zone trips on an open circuit; a normally open (NO) zone trips on a closed circuit. Supervised alarm zones use the NC configuration with an end-of-line resistor.

The EOL resistor value must equal the zone value programmed in the panel (common values are 1 kΩ, 2.2 kΩ and 4.7 kΩ); a mismatch produces a permanent trouble or a false alarm.

Class 2 and Class 3 circuits are power-limited and covered by NEC Article 725; fire alarm circuits by Article 760; communications circuits by Article 800.

Fail-safe (powered-to-lock) hardware unlocks when power is lost; fail-secure (powered-to-unlock) hardware stays locked when power is lost and is used where egress is released by a request-to-exit device.

A 4–20 mA analog loop uses 4 mA as 0 % (live zero) and 20 mA as 100 %; a reading below 4 mA indicates an open loop or a failed transmitter.

Sound pressure adds logarithmically: a 6 dB increase is roughly double the acoustic pressure, and a 10 dB increase is roughly double perceived loudness.

Cat5e/Cat6 horizontal channel is limited to 100 m (328 ft) including all patch and equipment cords; the permanent link is limited to 90 m.

RS-485: up to about 1200 m (4000 ft) of twisted pair, daisy-chain topology, no star branches, and 120 Ω termination at both physical ends of the bus.

RS-232: approximately 15 m (50 ft) maximum at 9600 bps.

T568B pin order, pins 1–8: white-orange, orange, white-green, blue, white-blue, green, white-brown, brown.

PoE budgets: 802.3af provides 15.4 W at the PSE and 12.95 W at the PD; 802.3at (PoE+) provides 30 W at the PSE and 25.5 W at the PD.

Standby battery capacity: Ah = (standby current × standby hours + alarm current × alarm hours) × 1.25 aging/derating factor.

Maintain at least 2 in. (50 mm) of separation between low-voltage/communications cable and power conductors; where separation is impossible, cross at 90°.

Voltage drop on a two-wire run: Vd = I × (2 × L × R), where R is conductor resistance per unit length and L is the one-way length.

Video storage per camera: GB/day ≈ bitrate in Mbps × 10.8; total storage = per-camera value × camera count × retention days.

Sealed lead-acid standby batteries typically serve 3–5 years; a load-test capacity below 80 % of rating indicates replacement is due.

A dynamic (under-load) battery test is required because a float-voltage reading alone will not reveal a failed cell.

Cross-zoning (alarm verification) requires two detectors in the same area to trip within a defined window before an alarm is transmitted, reducing false dispatches.

NTP synchronization keeps panel, recorder and access-control logs on a common time base; clock drift breaks incident correlation.

The daily automatic test signal confirms the communication path (dial-capture, IP or cellular); a missing test signal is reported as a communication trouble.

Camera image faults map to specific controls: blur to back focus, washout to exposure/WDR, banding to the anti-flicker 50/60 Hz setting, smearing to bitrate or I-frame interval.

Maintenance entries must record device ID, test method, result, technician and date to remain traceable.

Measure voltage with the circuit energized and the meter in parallel; measure resistance only with power removed and the component isolated.

Zone diagnostics: near 0 Ω indicates a short, an infinite reading indicates an open, and a reading offset by the EOL value places the fault on the panel side of the resistor.

Half-split method: divide the circuit at its midpoint, determine which half contains the fault, and repeat - a fault is located in log₂(n) tests.

A time domain reflectometer reports the distance to an open, short or impedance mismatch, so cable damage can be located without destructive access.

Cat6 certification covers wire map, length, insertion loss, NEXT and return loss; a pass requires every parameter to be within the standard's limits.

A ground fault on a supervised loop is found by measuring resistance from each conductor to earth; the low reading identifies the compromised leg.

Smoke detectors are functionally tested with calibrated aerosol or test gas and must annunciate at the panel; heat detectors are tested with a heat source or manufacturer magnet test.

IP device no-response order: verify power and link LEDs, ping the address, check for IP conflict or VLAN error, then substitute a known-good device.

NFPA 72 governs fire detection and alarm system installation, testing and inspection; NFPA 731 governs premises security system installation.

NEC Article 760 covers fire alarm cable (FPL, FPLR, NPLF); Article 725 covers Class 1/2/3 remote-control and signal circuits; Article 800 covers communications cabling.

Cable installed in air-handling (plenum) spaces must be listed CMP; riser cable is CMR; general-purpose is CM/CMG.

OSHA 1910.147 requires lockout/tagout of all hazardous energy before servicing, verified by an attempted start; NFPA 70E sets arc-flash PPE and approach boundaries.

Fall protection is required in general industry at 4 ft (1.2 m) and in construction at 6 ft (1.8 m).

Ladder setup follows the 4:1 rule - 1 ft of base setback for every 4 ft of working height - with the ladder extending 3 ft above the landing.

Every penetration of a rated wall or floor must be firestopped with a listed system that restores the assembly's fire resistance rating.

NFPA 101 requires egress doors to release on fire alarm signal; fail-safe locks must be tied to the fire alarm system and to a manual release.

The Record of Completion (NFPA 72) documents the installed system, device list and test results, signed by the installing contractor and the owner's representative.

As-built drawings must reflect field conditions: device addresses, zone schedules, cable identifiers, IP address tables and panel locations.

The battery calculation sheet, voltage-drop calculations and device test records are retained in the job file.

A punch list records every open deficiency with an assigned owner, a due date and a closure signature; the job is not complete until all items are closed.

Handoff training is documented with date, topics covered, attendee names and signatures.

Default installer and user passwords are changed at handoff and the new credentials are recorded in the secured handover document.

Warranty terms, service contact information and the preventive-maintenance schedule are delivered in writing as part of the closeout package.