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Peter Baarssen talks about inrush current

Piet Brouwer Electrotechnology, is a global specialist in turnkey electrical engineering for marine installations on both commercial vessels and yachts. When building, refitting or retrofitting any type of vessel, it is ideal to have a single experienced partner for the entire electrical system. Piet Brouwer Electrotechnology is a true one-stop shop, and provides a comprehensive package that consists of the design and installation of the electrical system, as well as of the monitoring, control and navigation & communication equipment.

Moreover, the company ensures that the onboard technology catches the eye primarily by being invisible – the systems are reliable and do their job quietly. A variety of transformers are installed as part of an onboard electrical system. Peter Baarssen, projectmanager at Piet Brouwer Electrotechnology, is an expert on the subject. 

Isolation transformers are commonly used on boats to create a separation between the electrical systems onboard and onshore. Transformers are also used to lower voltage – from 690 to 400 volts, say. While their actual application may vary considerably, these transformers operate according to the same principle.

A magnetic field is generated by leading an electrical current through copper coils mounted around a soft-iron core. The level of current depends on the size of the transformer and is referred to as normal current. The transformer is protected within the electrical system via a safeguard set to correspond to the level of this normal current. When a transformer is switched on, however, this can lead to very high peak current – much higher than normal current. This can result in the activation of the breaker and can, at worst, lead to much, or most of the system ending up without power. Obviously, this needs to be avoided.

There are a number of factors that affect the magnitude of the inrush current. They are linked to the size of the transformer, the moment when it is switched on and the presence of any residual magnetism in the transformer. To counteract these adverse effects, Piet Brouwer Electrotechnology places an electrical circuit in the transformer which pre-magnetises it.


The pre-magnetising process works as follows. The transformer and the load of the transformer are linked by relays. However, while the transformer is starting up, Piet Brouwer Electrotechnology ensures that the load is disconnected. Resistors are activated on the primary side of the coils, their capacity and type depending on the transformer. After the resistors are switched off, the supply current is turned on. The transformer is now magnetised, which limits the inrush current to less than the normal flow. After the magnetisation, which lasts a few seconds, the resistors are turned off again. The transformer is now ready to provide full power. After this, the relay linking to the load can be shut down. Pre-magnetising therefore prevents excessive peaks during the activation and safeguards the selectiveness of the protection measures.

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