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RCCBs pick up smooth DC earth fault currents

A Siemens Automation and Drives product story

Edited by the Engineeringtalk editorial team Aug 12, 2002

A new range of residual current circuit breakers incorporates a second detection circuit specifically for handling the smooth DC earth fault currents that can be produced by variable speed drives.

Variable Speed Drives (VSDs) are often an excellent solution to the problem of how to easily and efficiently control motions in a wide variety of machinery, including critical medical equipment such as X-ray machines and CT scanners.

However, there is one important aspect of their deployment that is often overlooked: earth leakage protection.

The smooth DC voltage element in a VSD means that standard RCCBs (residual current circuit breakers) do not give full protection in this application, even though they are highly suitable for detecting AC or pulsating DC fault currents.

Unfortunately, the RCCBs are unable to detect smooth DC fault currents and, indeed, it is probable that the smooth DC current will inhibit the tripping function of the pulsating DC detection circuit.

Medical equipment is often installed with a UPS (uninterruptible power supply) to ensure that it does not immediately cease functioning in the event of a main power supply failure.

However, because of their use of VSDs, such equipment should also be protected by a suitable earth leakage protection device.

Although conventional RCCBs are unsuitable for the reasons outlined above, there are some new units available that have a second detection circuit specifically for handling smooth DC earth fault currents.

Two examples are the Siemens 5SZ3 4 and the 5SZ6 4, both of which are available with 25A (11kW), 40A (18.5kW) and 63A (30kW) ratings.

These two models of RCCB offer alternative levels of protection and it is important that the correct device is specified.

While the 5SZ6 4 has a tripping current of 300mA and will therefore prevent fires starting due to earth faults, it will not protect personnel.

For this greater level of protection, the 5SZ3 4 is necessary because it has a tripping current of 30mA.

Neither device will provide short circuit protection, so an additional overcurrent device must still be fitted, such as a fuse, MCB (miniature circuit breaker) or MCCB (moulded case circuit breaker).

A further characteristic of VSDs that designers are not always aware of is that earth leakage currents often exceed 30mA during normal running because of the use of mains filters.

This presents a dilemma: should the filter be omitted, which could lead to problems of electrical interference with other equipment (not an attractive choice in a medical environment), or should the RCCB be dispensed with, resulting in a lowering of safety standards (also an unattractive proposition)? To help overcome this difficulty, Siemens has recently launched the MM4 range of VSDs that feature mains filters with an unusually low earth leakage current of 3.5mA, well below the 30mA tripping current of a 5SZ3 4 RCCB with secondary circuitry for detecting smooth DC earth fault currents.

Class B filters with low discharge currents are available for portable systems or where several VSDs up to 2.2kW single phase are fed via one RCCB.

Thanks to the new generation of RCCBs and VSDs from Siemens, designers and installers of equipment - especially those working in the medical field - can be sure of providing high performance motion, yet without compromising safety through presenting users with hazards caused by earth leakage faults or electrical interference.