mcmurdo E3 Owner's Manual page 5

the HRU cuts a plastic rod that holds back the spring and the lid is ejected,
releasing the EPIRB which floats to the surface and switches on automatically.
The diagram below illustrates the automatic release sequence.
1) As the vessel sinks, the enclosure fills with water. The HRU contains a blade
which is released due to water pressure acting on a diaphragm. Before it
reaches a depth of 4 metres, the HRU will operate and cut the plastic rod,
releasing the coil spring.
2) The spring pushes the E3 and the enclosure lid outwards. As the lid pivots off
it disengages from the screw head that helped hold it in place. The lid is
weighted so it rolls over and falls away.
3) As the E3 floats away from the lid, it moves out of range of the magnet. Once
away from the magnet its sea switch becomes armed.
4) The sea switch activates. The EPIRB then floats on the surface with its strobe
light flashing. After 50 seconds it makes its first distress transmission.
H
R
U
1) HRU cuts rod
If you need to manually activate your E3, it can be freed from the enclosure simply
by pulling out the R-shaped retaining pin and removing the lid.
2.3 Manual E3 in enclosure
Customers who need to mount their E3 in an exposed position can select the
protective enclosure without an HRU fitted (E3c). This means that the E3 will not
float free if your vessel sinks. This is known as a category 2 EPIRB. You can tell
which enclosure you have by reading the category from the front of the enclosure...
E3a Category 1
E3c Category 2
Magnet
2) Lid ejects
HRU fitted
No HRU fitted
3
4) sea switch
activates
3) EPIRB arms once
away from magnet
Automatic float-free
Will not float free