Page 149 - Offshore Electrical Engineering Manual

136 CHAPTER 13 Subsea Supplies and Cathodic Protection

DIVING CHAMBERS FOR SATURATION DIVING

As the lives of divers may depend on the electrical supply being available at the
chamber, particularly if deep diving involving compression chambers is being under-
taken, the electrical supply systems of the associated support vessels must be of the
highest integrity and reliability.
The equipment used inside diving chambers is, where possible, to be designed
using the principles of hazardous area ‘intrinsic safety’.
This is for the following reasons:

1. To prevent ignition should flammable well gases be present
2. It will help to prevent fires in a chamber with an oxygen enriched atmosphere
3. It will ensure earth fault currents are limited to very low values.

Table 2.13.1 shows the effects on the human body of various magnitudes of ac cur-
rent and from this it can be seen that values beyond 18 mA are dangerous. In salt water
resistances tend to be low and very low voltages are required to achieve this threshold
value. All electrical circuits within the chamber will need to be protected against earth
faults by using current operated earth leakage circuit breakers set to trip at not more
than 15 mA in 30 ms. The power supply within the chamber should be low voltage and

Table 2.13.1 Physiological Effects of Electricity on Humans
Current
at 50 to Duration of
60 Hz rms mA Shock Physiological Effects on Humans

0–1 Not critical Range up to threshold of perception, electric current
not felt.
1–18 Not critical Independent release of hands from objects gripped
no longer possible. Possibly powerful and sometimes
painful effect on the muscles of fingers and arms.
18–30 Minutes Cramp-like contraction of arms. Difficulty in breathing.
Rise in blood pressure. Limit of tolerability.
30–50 Seconds to Heart irregularities. Rise in blood pressure. Powerful
minutes cramp effect. Unconsciousness. Ventricular fibrillation if
long shock at upper limit of range.
Less than No ventricular fibrillation, heavy shock.
cardiac cycle
50 to a few Above one Ventricular fibrillation. Beginning of electrocution in
hundred cardiac cycle relation to heart phase not important. (Disturbance
of stimulus conducting system?) Unconsciousness.
Current marks.
Less than Ventricular fibrillation. Beginning of electrocution in
cardiac cycle relation to heart phase important. Initiation of fibrillation
only in the sensitive phase. (Direct stimulatory effect on
heart muscle?) Unconsciousness. Current marks.
Above a few Over one Reversible cardiac arrest. Range of electrical defibrilla-
hundred cardiac cycle tion. Unconsciousness. Current marks. Burns.