Handbook for Sound Engineers

Transmission Techniques: Wire and Cable 443

tors pass through a toroidal (differential) current trans-

former. When everything is operating properly, the

vector sum of the currents is zero. When the currents in

the two legs are not equal, the toroidal transformer

detects it, amplifies it, and trips an electromagnetic

relay. The circuit can also be tested by depressing a test

button which unbalances the circuit.

14.28.7 AC Power Cords and Receptacles

Ac power cords, like other cables come with a variety of

jacket materials for use in various environments. All

equipment should be connected with three-wire cords.

Never use ground-lift adapters to remove the ground

from any equipment. This can be dangerous, even fatal,

if a fault develops inside the equipment, and there is no

path to ground.

A common European plug, with a rating of 250 Vac

and 10 A, is shown in Fig. 14-27.

The color codes used in North America and Europe

for three conductors are shown in Table 14-38.

Cables should be approved to a standards shown in

Table 14-39.

The UL listing signifies that all elements of the cords

and assembly methods have been approved by the

Underwriters Laboratories, Inc. as meeting their appli-

Table 14-37. Physiological Effects of Shock Current
on Humans. From Amundson.

Shock

Current in

mArms

Circuit

Resistance

at 120 Vac

Physiological Effects

0.5–7 mA 240,000:

down to

17,000:

Threshold of Perception: Large

enough to excite skin nerve endings for

a tingling sensation. Average thresholds

are 1.1 mA for men and 0.7 mA for

women.

1–6 mA 120,000:

down to

20,000:

Reaction Current: Sometimes called

the Surprise current. Usually an invol-

untary reaction causing the person to

pull away from the contact.

6–22 mA 20,000:

down to

5400 :

Let-Go Current: This is the threshold

where the person can voluntarily with-

draw from the shock current source.

Nerves and muscles are vigorously

stimulated, eventually resulting in pain

and fatigue. Average let-go thresholds

are 16 mA for men and 10.5 mA for

women. Seek medical attention.

15 mA and

above

8000 : and

below

Muscular Inhibition: Respiratory

paralysis, pain and fatigue through

strong involuntary contractions of mus-

cles and stimulation of nerves. Asphyx-

iation may occur if current is not

interrupted.

60 mA–5 A 2000:

down to

24 :

Ventricular Fibrillation: Shock cur-

rent large enough to desynchronize the

normal electrical activity in the heart

muscle. Effective pumping action

ceases, even after shock cessation.

Defibrillation (single pulse shock) is

needed or death occurs.

1A and

above

120 : and

below

Myocardial Contraction: The entire

heart muscle contracts. Burns and tis-

sue damage via heating may occur with

prolonged exposure. Muscle detach-

ment from bones possible. Heart may

automatically restart after shock

cessation.

Figure 14-26. Typical ground-fault circuit interrupter.

Figure 14-27. Standard international plug.

Table 14-38. Color Codes for Power Supply Cords

Function North America CEE and SAA Standard

N—Neutral White Light Blue

L—Live Black Brown

E—Earth or

Ground

Green or

Green/Yellow

Green/Yellow

Table 14-39. Approved Electrical Standards

Country Standard

United States UL Underwriters Laboratory

Canada cUL Canadian Underwriters Laboratory

Germany GS/TUV German Product Certification

Organization

International IEC International Electrotechnical

Commission

Line in

H H

N N

Gnd

Amplifier

Toroidal

Relay

Gnd

Test

Line out/

load