Guard and shielding

Modern PCs and peripherals often employ an assortment of metal and plastic shields or guards within the device. Shields and guards serve a variety of purposes, but all should be replaced when service is complete.

1. Replace EMI shields. PCs operate at very high frequencies, and the signals they generate can sometimes be transmitted to nearby recivers such as radios and televisions. Ideally, the PCs design should prevent such EMI (electromagnetic interference), but it may also be necessary to add metal shields to attenuate excessive interference. Whenever you remove metal housings or shields from a PC, be sure to replace them before returning the system to service. 
2. Replace x-ray shields. Monitors use extremely high voltages at the CRT, which in turn can liberate x-rays through the CRT glass. The lead contained in CRT glass is usually a sufficient shield, but larger CRT funnel. When you remove x-ray shields from a monitor, be sure to replace them before running the monitor or returning it to the service. 
3. Replace all guards and other mechanical assemblies. Printers typically employ a large assortment of guards and covers (both plastic and metal) to protect delicate mechanical assemblies from dust and accidental contact. You can usually operate a printer for short periods without guards in place, but you should always make in a point to replace any protective assemblies before returning the device to service.