I suggest you search for a beginning electronics book, or course to take.

It will be better than playing 20 questions here.

• Fundamental Electricity - Scribd
• Amazon.com: Fundamentals of Electricity: Basic Principles v ...
  
ELECTRICITY FUNDAMENTALS
1. Amazon.com: Fundamentals of Electricity: Basic Principles v ...
1. ELECTRICITY FUNDAMENTALS

Voltage ELCB sense unsafe levels of voltage (old school ELCB's), Current driven ELCB's (like RCD's) detect current imbalance. While still termed as ELCB's, most (if not all) ELCB's are current operated.

RCDs are Residual current devices, it is a category that encompasses all ELCB's, RCCDs, RCBOs, ELRs, etc...

earth leakage (ELCB) detects a voltage on the earth and assumes a fault.

An RCD is the difference between the current in the phase and neutral conductors. If the RCD or RCCB detects an imbalance it assumes a fault.

An ELCB requires 3 wires and RCD only 2 hence 'safety life saver' type trips on double insulated lawnmowers and hedge cutters.

ELCB is very prone to spurious faults from adjacent systems connected to your earth and to the best of my knowledge are banned from general use in the UK. Without an earth connection i.e. a double insulated piece of equipment it would not offer protection.

Your absolutely correct, only need to note that ELCBs are not necessarily volt-detecting leakage devices. Although you're right, they have started out as such, but up-to-date, manufacturers still produce current detecting leakage breakers identified as ELCBs. Volt-ELCBs maybe banned in the UK as you stated but I'm yet to see a country that utilize them anymore; only to emphasize their decommission in most countries as well if not worldwide.