Electrical Installation Safety
Trip-lock RCD
A method of performing a loop imped- ance measurement on RCD protected installations without tripping the RCD. All Metrel’s multifunctional installation testers have the non trip facility.
Tip commander
An electronic remote to help make con- tinuity and insulation testing easier by placing the test button and either mem- ory or backlight button in the hand of the electrician. Available as a 2-wire tip com- mander or 3-wire tip commander that al- lows loop impedance and RCD testing.
Time delayed (S-type) RCDs In situations where multiple RCDs oc- cur in an installation the need to dis- criminate or coordinate when the RCDs will trip has become more important. If a 100 mA RCD and a 30 mA RCD are protecting the same circuit (e.g. one at source and one on the individual circuit) and a fault above 100 mA occurs, it may not always be the case that the 30 mA RCD trips first (e.g. the 100 mA RCD could have a faster response time in the case of a fault). In this situation, a time delayed or selective (S-type) RCD is re- quired at the source of the installation so that the 30 mA RCD has time to trip and, if the problem is caused by a none RCD protected circuit, the supply is still safely disconnected.
Not all installation test instruments have the ability to check time delayed RCDs. Therefore as RCDs become more com- monplace, it is useful to have a test instrument that has the ability to test them. All Metrel’s multifunctional test instruments and single function live cir- cuit testers have the ability to test both general and selective RCDs.
Online voltage monitoring The online voltage monitoring function built into all of Metrel’s MI 3000 series multifunctional installation testers. This function displays on one screen the AC voltages and frequency occurring be- tween L to PE, L to N and N to PE (single
Accessories: page 1.48
phase systems) and L1 to L2, L2 to L3 and L3 to L1 (3-phase systems).
This feature is very useful for fault find- ing on systems e.g. quickly identifying incorrect connections, disconnected wires and incorrect voltages.
CAT I CAT II CAT III CAT IV
The higher the CAT rating of your test in- strument, the more protection it will give you in the case of a fault occurring on the system under test (e.g. CAT IV / 300 V installation test instrument provides significantly more protection to the user in the case of a fault than a CAT III / 300 V installation test instrument).
Safety CAT Ratings Transients are very fast, high energy spikes that can occur on the mains pow- er supply. Low energy transients can be caused by simply turning on a switch to a circuit or electromagnetic interfer- ence while high energy transients can be caused, for example, by a powerful motor stalling or a lightning striking a power line.
Transients can have a variety of effects which could include blowing the protec- tive fuse in the appliance, causing light bulbs to blow, causing insulation be- tween conductors to break down and, in the case of high energy transients caus- ing appliances connected to the supply to set on fire or produce dangerous sparks.
The less protection provided against these transients enables higher en- ergy transients to occur (i.e. if lightning strikes the power lines, you would not expect the full fault voltage to occur at the power supply socket on the wall in your house). The level of danger due to transients is therefore divided into cat- egories. This is illustrated in the follow- ing diagram:
1. 3
All Metrel’s MI 3000 series multifunc- tional installation test instruments are rated CAT IV / 300 V. CAT IV / 300 V means that the instrument is suitable for testing up to CAT IV locations up to 300 V between line and earth and, due to the relationship set out in IEC/EN 60364, testing in CAT III locations up to 600 V between line and earth.
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