PARALLEL OF UPS AND RESIDUAL-CURRENT DEVICE (RCD)

In case of plants with UPSs in parallel and the use of RCD protections, also known as Differential Protections, some spurious tripping have been reported.

The most frequent cause can be explained as follows:

The neutral is passing through the UPSs
Although apparently the same, the two systems neutral impedance can never be identical
The leakage current of one system re-closes, therefore, through the two neutrals according to a division which could lead to different current distributions on the neutrals (see Figure 1).

Figure 1 – Unproper RCD installation (IN1≠IN2 where IN1+IN2=IN)

To solve this problem, it is advisable to fit a single circuit breaker upstream of the static UPS units (see Figure 2) or equip each unit with a circuit breaker with no further re-coupling downstream. Essentially, the loads should be divided between the two different units, by creating a total and distinct parallel