HP Z4 Maintenance and Service Guide - Page 17

3 Routine care, SATA drive guidelines, and disassembly preparation This chapter provides general service information for the computer. Adherence to the procedures and precautions described in this chapter is essential for proper service. CAUTION: When the computer is plugged into an AC power source, voltage is always applied to the system board. The power cord must be disconnected from the power source before opening the computer to prevent system board or component damage. Electrostatic discharge information A sudden discharge of static electricity from your finger or other conductor can destroy static-sensitive devices or microcircuitry. Often the spark is neither felt nor heard, but damage occurs. An electronic device exposed to electrostatic discharge (ESD) may not appear to be affected at all and can work perfectly throughout a normal cycle. The device may function normally for a while, but it has been degraded in the internal layers, reducing its life expectancy. Networks built into many integrated circuits provide some protection, but in many cases, the discharge contains enough power to alter device parameters or melt silicon junctions. Generating static The following table shows how humidity affects the electrostatic voltage levels generated by different activities. A product can be degraded by 700 volts. ● Different activities generate different amounts of static electricity. ● Static electricity increases as humidity decreases. Event Walking across carpet Walking across vinyl floor Motions of bench worker Removing DIPs from plastic tube Removing DIPs from vinyl tray Removing DIPs from Styrofoam Removing bubble pack from PCB Packing PCBs in foam-lined box Relative Humidity 55% 7,500 V 3,000 V 400 V 400 V 2,000 V 3,500 V 7,000 V 5,000 V 40% 15,000 V 5,000 V 800 V 700 V 4,000 V 5,000 V 20,000 V 11,000 V 10% 35,000 V 12,000 V 6,000 V 2,000 V 11,500 V 14,500 V 26,500 V 21,000 V Electrostatic discharge information 9