Question 7

Question: How should modules “MUSH-DL” be used for the connection of two-wire detectors SPD-2.1, SPD-3, SPD-3.1, SPD-3.1M to a control panel with the 4-wire scheme of connection?

Answer:

It is possible to organize the functioning of modules in 2 modes:

• Nominal load;
• Maximum load;

The open-circuit in any place of alarm loops shall be interpreted as a fault in the nominal load mode.

The summary consumption current of detectors must not exceed the threshold of a module’s open-circuit. The maximum quantity of detectors is determined by the formula:

N_det = Ibr / I_j (1)

Where N_det — maximum quantity of detectors in an alarm loop; I_br — threshold of determination the status “Open-circuit” by “MUSH-DL”; I_j — consumption current of a detector in the standby mode.

For serially manufactured detectors SPD-2.1 and for a module MUSH-DL:

I_br = 1,6 mA

I_j = 0,22 mA

N_det = 1,6/0,22 ≈ 7 pcs.

For serially manufactured detectors SPD-3, SPD-3.1, SPD-3.1M and for a module MUSH-DL:

I_br = 1,6 mA

I_j = 0,09 mA

N_det = 1,6/0,09 ≈ 17 pcs.

It is recommended to place such a terminal resistor in the nominal load mode so that the summary current drawing through R_term and consumed by detectors would be within the limits of 4 mA, that is:

I_al = U_al / R_term + I_sm * N_det (2)

Where I_al — summary current in an alarm loop; U_al — nominal voltage in an alarm loop; I_det — summary consumption current by detectors in the standby mode; I_sm — consumption current of a detector in the standby mode. N_det — quantity of installed detectors in an alarm loop;

Thus:

R_term = U_al / (I_sm - I_sm * N_det)

R_term = 12V / (4mA - 0,22mA * 7) ≈ 4,7 KΩ

The limiting resistor in the electric circuit of a detector must be chosen, so that the current increase in AL equals approx. 5 mA when the detector activates. When the internal resistance of the detector in the “Fire” mode equals approx. 500 Ω, the resistance value of the limiting resistor must equal R_term = 12В/5mA-0,5 KΩ ≈ 2 KΩ