Site issues with Earth leakage
Abstract
General:
This document examines one of the most common site issues with UPS systems,
which is the tripping of the earth leakage protection, Residual Current Device (RCD)
also known as an Earth Leakage Circuit Breaker (ELCB).
Technical Description:
One of the most common site issues with UPS systems is the tripping of the earth
leakage circuit breakers ELCB / RCD. To examine this in greater detail, we need to
look at this device:
ELCB’s contain a core-balance transformer and work on the following simple
principal: If the current which goes out on the phase does not return on the neutral, or
on another phase, in the event of a three phase type, it will be detected as an earth
fault through an imbalance in the core balance transformer and this imbalance will
activate a tripping coil so that the circuit will be disconnected. Therefore, if one
comes in contact with a live appliance, the current, which flows through this person to
return to earth, is sufficient to trip a 30mA RCD instantly.
Principles of Operation of a Standard RCD / ELCB
Schematic diagram of RCD connected to earth fault showing principle parts of installation.
COIL to detect
Most Common Problems:
When a customer calls with a problem with a tripping RCD, one should never assume
that our unit is faulty without doing the required troubleshooting.
1) The EN50091 permits a leakage of up to 3.5 mA per pluggable appliance. If the
customer has 10 appliances connected with an average 3 mA per appliance, we can
have units tripping even without a faulty appliance. We should try and ascertain if it
still trips when the UPS is connected on it’s own, if possible on a dedicated RCD.
If this is the case, we should replace the unit, if not we should ask the customer to
have his installation and other appliances checked.
2) There are also issues when power electronics are connected. Some SMPS loads
cause high leakage after being switched on. In such cases we should recommend a
selective type of RCD. These RCD’s have a slight delay, which allows them to
overcome this relative short period of earth leakage.
3) In a new electrical installation RCD’s may trip due to an error in the wiring, such
as neutral wires being mixed up at distribution board
Tripping Currents
The most common tripping current for an RCD is 30mA but there are also types with
a tripping of 10mA and 100mA tripping current to offer shock protection.
There is also a 300mA and 500mA, which gives overall protection against risk of fire
from electrical faults in wiring etc. These are usually used at the main distribution
board in a TT network, where the main fuse may not react fast enough in the event of
a short circuit to earth.