Don't assume a switch or circuit breaker has cut off the electric power. Before you touch any bare wires, with hands or tools, check the wires with a 240-volt neon test light. First be sure the test light is working by inserting its prods into a receptacle. It should glow. Then open the access cover to the heater junction box, removing the wire nuts or tape from the heater's electrical connections carefully without touching them. Contact the test prods between the heater's green grounding screw or other reliable ground and each of the exposed wires in turn (Fig. 12). Furthermore, touch the prods across the bare wires themselves. If the test light glows at any time, at least some of the wires are still "live," indicating that power to the heater is not off. Call in an electrician before proceeding on your heater change. This is critically important, because electricity, water, and plumbing create a potentially hazardous combination. Only when you know the wires are dead is it safe to disconnect the electrical cable serving the heater and remove it.
Do not attach aluminum wires to copper ones inside the heater.
See that the heater is properly grounded, electrically (Fig.13).
Keep out of the reset and thermostat section of the heater, as they contain exposed live wiring.