Electrical switching apparatus exploded and burned yesterday in the Washington Hilton Hotel, injuring at least nine persons, most of them electricians trying to repair damage caused Saturday by a similar incident that forced the sold-out hotel to close.

Officials of the 1,154-room hotel at Connecticut Avenue and T Street NW had said they planned to reopen yesterday, but after the 12:17 p.m. explosion they said they did not know when guests could return.

Yesterday's blast occurred in what a hotel spokesman called an electrical switchboard located in the same underground room where a similar piece of equipment blew up and caused a three-alarm fire Saturday.

Hotel manager William H. Edwards was outside the building, giving an interview to a television crew when yesterday's explosion occurred. Soot-streaked electricians suddenly began streaming from an exit, joined by hotel employes who shouted and called warnings to bystanders.

Yesterday's blast, like the one the previous day, drew scores of firefighters and police to the scene and tied up traffic on thoroughfares near the hotel. Total damage was placed at about $500,000. The cause of the explosions was not known and an investigation was still under way late yesterday.

One of the injured persons was in serious condition last night. The others' injuries appeared minor. No guests were injured, either yesterday or Saturday.

Most of the 3,800 people who had been housed at the hotel are teen-agers attending a religious conference at the Washington Convention Center. They spent Saturday night at the Hilton's expense at 19 other Washington hotels and were expected to be lodged last night in at least 24.

Although many of the youths and their adult leaders who came here for the Youth Congress '85 convention had gone without a change of clothes for some time, they appeared to be taking the disruptions in stride.

"So it's another night with the same socks," a 31-year-old convention chaperone said yesterday as he approached the 12-story hotel in the hope of getting access to his room and learned of the second explosion.

Both blasts occurred in an electrical equipment room located in the south wing of the hotel, three stories below the main lobby. Mike Moscati, an employe of the Haislip Corp., an electrical contractor, was in the room yesterday when the blast occurred.

"It was pretty wild," he said. "There was a lot of smoke. As soon as I heard the sparks I took off."

Moscati said he heard the sound of an electrical arc, which is often a sign of a serious failure or malfunction in heavy-duty electrical equipment.

"I didn't hang around," Moscati said. "I've heard that sound before."

The electricians, many of them in T-shirts bearing the name of their company, burst from the entrance on the south side of the building.

One of them, apparently the most seriously injured, seemed to be in shock. His hair was singed and his face reddened from an apparent burn.

Rescue workers administered oxygen before the man was taken away by ambulance.

He was later admitted to the burn unit of the Washington Hospital Center where he was identified as William Hensen and listed in serious condition with first-degree facial burns.

At least seven of the eight other injured persons, who included a woman hotel employe, were treated at hospitals for smoke inhalation. One man remained last night at Howard University Hospital in good condition. The eighth person was released from Georgetown University Hospital after treatment; his injury was not disclosed.

When Saturday's fire broke out most of the hotel's guests were at the convention center. Yesterday only electricians and 50 to 100 employes were in the building.

Rather than transformers, as they have occasionally been described, the devices that blew up are actually known as switchboards, according to an electrician familiar with the equipment. He said they handle current coming into the hotel at 480 volts from transformers outside.

The outside transformers receive electricity at 13,000 volts and step it down to 480, according to the electrician.

Lower-voltage transformers located inside the building receive current from the switch gear and step it down still further, as needed, he said.

"We don't really know what caused it," D.C. Fire Chief Theodore R. Coleman said of yesterday's explosion. Other fire officials said it was believed both explosions had similar causes, but further details could not be learned.

Questions also continued to linger yesterday about timing and procedures followed in sounding internal alarms and evacuating the hotel after Saturday's fire.

After firefighters went into the building Saturday, they delayed entering the electrical room in which the fire broke out for two hours until they were assured the equipment was not cooled by polychlorinated biphenyls (PCBs), which emit deadly fumes when they burn.

Some guests have said they neither heard alarms nor were evacuated for some time after the fire broke out.

Betty Martin of Nashville, who came here for the Youth Congress with her son Daren, 15, said they were told by another guest about 10:30 a.m. Saturday that fire engines were at the building.

She said her son got trapped in an elevator on the eighth floor, and when she went down a staircase she found it "filled with very heavy fumes and some smoke."

She said that firefighters extricated her son about 11:10, but that before an alarm was sounded about 11:30, no one warned of danger or said "get out of the hotel."

Fire Chief Coleman yesterday supported the position taken earlier by Edwards, the hotel manager, who said no smoke was seeping into upper levels of the building and there was no danger of the fire spreading.

"Maybe there was a small amount of smoke in some upper levels through the ventilation" system, Coleman said, "but most of it was below the lobby level . . . . The fire was three floors below the lobby level."

He said the interval that elapsed between the outbreak of the fire and the order to evacuate was "not a long time."

"In my opinion," Coleman said, "there was no real danger to the guests."

He also said that in the two hours before obtaining confirmation that no PCBs were involved in the Saturday blaze, firefighters "were not just standing around."

Instead, he said, they worked to contain the fire within the electrical equipment room and helped in the evacuation.

Hilton officials said no smoke from yesterday's fire rose into the guest areas of the hotel.

Coleman said, however, that high levels of carbon monoxide were measured in the basement, and that they would have to be reduced before guests would be allowed back into the hotel.

Hilton officials said they will announce today when the hotel might reopen.

More than 100 firefighters using 50 pieces of equipment went to the hotel yesterday in response to the explosion. The fire was extinguished by 1:20 p.m., fire officials said.

Efforts to exhaust smoke and fumes yesterday were hampered by the absence of electricity, the officials said.

The almost 3,800 conventiongoers who had been staying four to a room at the Hilton are among about 15,000 who are attending the meeting, which ends tomorrow.

The fire Saturday created a considerable logistical scramble before alternate accommodations were located for those displaced. For many of those who had been staying at the Hilton, a major inconvenience has been the inability to get access to their luggage.

However, the chaperone who was on his way to the hotel yesterday when the explosion occurred appeared relatively unruffled by the disruptions.

"Oh, well," said Sean O'Brien, 31, who came here with 53 Youth Congress participants from a Lutheran church in White Bear Lake, Minn., "we'll get by. Even in this we see how God is providing . . . . "

Last night some of the convention participants were permitted to enter the Hilton to obtain belongings before the hotel reopens officially.

Maintenance work by the Haislip Corp. in the underground electrical equipment room is expected to resume today, Hilton spokeswoman Renee Subrin said.