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Appen dix EPower Requirements for

Series Firing CircuitsE-1. Series Circu its. Electric blasting caps ar e connected in series and fried with an electric pow ersource (blasting machine). A series circuit provides a single path for the electrical current that flowsfrom one firing w ire, through each blasting cap to th e next blasting cap, and back to the other firingwire. A series circuit shou ld not contain m ore than 50 blasting caps. The connection of more th an50 caps in a ser ies circuit increases the chan ces of breaks in the firing line or cap lead s.

E-2. Ohms Law. Ohms Law defines the amou nt of voltage necessary to detonate th e blastingcaps. Determine the requ ired voltage for you r firing circuit as follows:

E=IR (E-1)

where

E = electric potential, or voltage, in volts.I = current, in amperes.R = resistance, in ohms.

E-3. Electric Power Formu la.Determine the am oun t of electric pow er (watts) necessary todetonate blasting caps:

W= I2R (E-2)

where

W = electrical power, in watts.I = current, in amperes.R = resistance, in ohms.

E-4. Electric Blasting Caps. Military electric blasting caps connected in series require at least 1.5amperes to fire, regardless of the number of caps in the series. The resistance of military electricblasting cap is 2 ohms.

E-5. Circuit Resistance. Ensure that the pow er source is adequate to fire all charges connected tothe circuit. Firing w ire, as well as blasting caps, contribu te to total resistance in the circuit.Determine the am oun t of resistance by combining the ind ividual resistances of the blasting capsand the wires. The resistance in the wire d epend s on the w ires size and length. Table E- 1 (page

E-2) gives the resistance per 1,000 feet of var ious sizes of copp er wire.

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E-6. Series Circuit Calculations. Complete calculations for any series circuit involved indetermining the amount of current (amperes), voltage (volts), and power (watts) needed to fire thecircuit. Use the following procedu re:

a. Current. The current requ ired for a series circuit of electric blasting cap s is 1.5 am per es,regardless of the number of blasting caps in the circuit.

b.Resistance. Determ ine the resistance in the circu it (parag raph E-5, pa ge E-l).

c. Voltage. Determine the r equired voltage for the circuit (paragrap h E-2, page E- 1).

d . Power. Determine the required power for the circuit (paragraph E-3, pa ge E-l).

e. Example. Determine the current, voltage, and power required to detonate a 20-cap seriescircuit consisting of special electric blasting caps and 500 feet of standard, 2-conductor, 18-gaugefiring w ire.

(1) Curren t. The amou nt of curr ent requ ired to detonate this circuit is 1.5 amp eres

(2) Resistan ce.

Cap s: 2.0 ohms (20 caps)= 40.0 ohm s

Wire: 500 feet (2 strands)= 1,000 feet= 6.4 ohms (Table E-1)

Total Resistance: 46.4 ohms

NOTE: Number-18 wire consists of two strands. The example specifies a 500-foot piece of wire,so u se 1,000 feet as th e total w ire length for determ ining resistance (500 x 2 = 1,000).

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(3) Voltage.

E = IR = 1.5(46.4)= 69.6 volts (E-3)

where-

E = voltage, in voltsI = current, in amperes

R = resistance, in ohms(4) Pow er.

W= 12(R)= 1.5

2(46.4)= 104.4 WattS (E-4)

where

W = power, in wattsI = current, in amperesR = resistance, in ohms

E-7. Voltage D rop . Ohms Law allows you to determine the amount of voltage required (voltaged rop ) for a blasting circuit. In p ractice, the voltage drop shou ld nev er exceed 90 percent of the

available voltage; if it does, decrease the resistance or increase the voltage in th e circuit to ensu rethat proper detonation occurs.

E-8. Blasting M achine s. The name plate on power sour ces normally states the amp erage and thevoltage ratings. Before u sing any pow er source, determine w hether it is su itable for your firingcircuit. Generally, you can d etermine the adequacy of a pow er source by consulting Table E-2 (pageE-4). This table lists the sizes of circuits that power sources can support. If you must determinethe power sources capabilities from the name plate, use the following procedure:

a.Determining Circuit Capacity.

Step 1. Multiply the power sources voltage rating by 90 percent to get an adjustedvoltage rating.

Step 2. Divide th e adjusted voltage ratin g (Step 1) by the circuits amperage ratin g (1.5amp eres). At this point you have th e maximu m allowable resistance in the circuit, inohms.

Step 3. Determine the total resistance from the firing wire (Table E-l).

Step 4. Subtract the wires resistance from the maximum allowable circuit resistance(Step 2) to determine the maximum allowable resistance of the blasting caps in the circuit.

Step 5. Determine the maximu m nu mber of blasting caps the circuit will supp ort bydividing the allowable resistance for caps (Step 4) by the resistance in one cap (2 ohms).

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b. Example. Determine the m axim um nu m ber of electric blasting caps allowed in a seriescircuit fired by a 220-volt, 13.5-amp ere genera tor and 250 feet of dou ble-strand , 20-gau ge wire (atotal of 500 feet of wire).

(1) Allowable Resistance.

0.90(220 volts) = 132 ohms1.5 amperes

(2) Resistan ce in Firing Wire.

10.2 ohms (500feet)1,000

= 5.2 ohms

(E-5)

(E-6)

(3) Allowable Resistance in Blasting Caps.

132 ohms 5.2 Ohms = 126.8 Ohms (E-7)(4) Number of Blasting Caps.

126.8 ohms

2 ohms= 63.4 caps (Round down to 63 caps) (E-8)

E-9. Batteries an d Dry Cells.Use the procedu re in paragrap h E-8 (page E-3) to determ ine thesize of a circuit supported by a battery or dry cell.

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