 Late-model ER2s also feature a ladder blocking relay which prevents the extension of maintenance ladders when the pantograph is raised, thus preventing workers and bystanders from climbing onto the roof.For passenger safety, all doors are equipped with position sensors which notify the engineer whether all doors are shut (however, in practice it is standard operating procedure for the fireman to step out onto the platform and check the doors visually).Other protective equipment includes the voltage relay (which trips if the voltage in the high-voltage circuit falls below 2400 V), the dynamotor/compressor overload relay, the heating circuit overload relay, and the automatic control shutoff switch.